WD-css1-960726
WD-css1-960726
This is a W3C Working Draft for review by W3C members and other interested parties. It is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress". A list of current W3C technical reports can be found at http://www.w3.org/pub/WWW/TR. Since working drafts are subject to frequent change, you are advised to reference the latter URL, rather than the URL for this working draft.
This document specifies level 1 of the Cascading Style Sheet mechanism (CSS1). CSS1 is a simple style sheet mechanism that allows authors and readers to attach style (e.g. fonts, colors and spacing) to HTML documents. The CSS1 language is human readable and writable, and expresses style in common desktop publishing terminology.
One of the fundamental features of CSS is that style sheets cascade; authors can attach a preferred style sheet, while the reader may have a personal style sheet to adjust for human or technological handicaps. The rules for resolving conflicts between different style sheets are defined in this specification.
Comments to this draft can be sent to the www-style@w3.org mailing list, or directly to the authors. For background material, see the resource page on web style sheets [1]. We encourage browser vendors to start implementing CSS1.
Abstract
Terminology
1 Basic concepts
1.1 Containment in HTML
1.2 Grouping
1.3 Inheritance
1.4 Class as selector
1.5 ID as selector
1.6 Contextual selectors
1.7 Comments
2 Pseudo-classes and pseudo-elements
2.1 Anchor pseudo-classes
2.2 Typographical pseudo-elements
2.3 The 'first-line' pseudo-element
2.4 The 'first-letter' pseudo-element
2.5 Pseudo-elements in selectors
2.6 Multiple pseudo-elements
3 The cascade
3.1 'important'
3.2 Cascading order
4 Formatting model
4.1 Vertical formatting
4.2 Horizontal formatting
4.3 Lists
4.4 The canvas
4.5 Floating elements
4.6 Replaced elements
5 CSS1 properties
5.1 Notation for property values
5.2 Font properties
5.2.1 Font encoding
5.2.2 'font-size'
5.2.3 'font-family'
5.2.4 'font-weight'
5.2.5 'font-style'
5.2.6 'line-height'
5.2.7 'font'
5.3 Color and background properties
5.3.1 'color'
5.3.2 'background'
5.4 Text properties
5.4.1 'word-spacing'
5.4.2 'letter-spacing'
5.4.3 'text-decoration'
5.4.4 'vertical-align'
5.4.5 'text-transform'
5.4.6 'text-align'
5.4.7 'text-indent'
5.5 Box properties
5.5.1 'margin-left', 'margin-right', 'margin-top', 'margin-bottom', 'margin'
5.5.2 'padding'
5.5.3 'border-top', 'border-right', 'border-bottom', 'border-left', 'border'
5.5.4 'width'
5.5.5 'height'
5.5.6 'float'
5.5.7 'clear'
5.6 Classification properties
5.6.1 'display'
5.6.2 'list-style'
5.6.3 'white-space'
6 Units
6.1 Length units
6.2 Percentage units
6.3 Color units
6.4 URL
7 CSS1 conformance
7.1 forward-compatible grammar
7.2 Parsing conventions
8 References
9 Acknowledgments
Appendix A: Sample style sheet for HTML 2.0
Appendix B: A sample lex/yacc grammar
Appendix C: The applicability and extensibility of CSS1
Designing simple style sheets is easy. One only needs to know a little HTML and some basic desktop publishing terminology. E.g., to set the text color of 'H1' elements to blue, one can say:
H1 { color: blue }
The example above is a simple CSS rule. A rule consists of two main parts: selector ('H1') and declaration ('color: blue'). The declaration has two parts: property ('color') and value ('blue'). While the example above only tries to influence one of the properties needed for rendering an HTML document, it qualifies as a style sheet on its own. Combined with other style sheets (one fundamental feature of CSS is that style sheets are combined) it will determine the final presentation of the document.
The selector is the link between the HTML document and the style, and all HTML tags are possible selectors. HTML tags are defined in the HTML specification [2], and the CSS1 specification defines a syntax for how to address them.
The 'color' property is one of around 35 properties that determine the presentation of an HTML document. The list of properties and their possible values is defined in this specification.
HTML authors only need to write style sheets if they want to suggest a specific style for their documents. Each User Agent (UA, often a "web browser" or "web client") will have a default style sheet that presents documents in a reasonable -- but arguably mundane -- manner. Appendix A contains a sample style sheet to present HTML documents as suggested in the HTML 2.0 specification.
In order for the style sheets to influence the presentation, the UA must be aware of their existence. Another W3C working draft, HTML3 and Style Sheets [4], describes how to link HTML with style sheets:
<HTML>
<HEAD>
<TITLE>title</TITLE>
<LINK REL=STYLESHEET TYPE="text/css"
HREF="http://style.com/cool" TITLE="Cool">
<STYLE TYPE="text/css">
@import url(http://style.com/basic);
H1 { color: blue }
</STYLE>
</HEAD>
<BODY>
<H1>Headline is blue</H1>
<P STYLE="color: green">While the paragraph is green.
</BODY>
</HTML>
The example shows four ways to combine style and HTML: using the 'LINK' element to link an external style sheet, a 'STYLE' element inside the 'HEAD' element, an imported style sheet using the CSS '@import ...' notation, and a 'STYLE' attribute on an element inside 'BODY'. The latter option mixes style with content and one loses the corresponding advantages of traditional style sheets.
The 'LINK' element references alternative style sheets that the reader can select, while imported style sheets are automatically merged with the rest of the style sheet.
Traditionally, UAs have silently ignored unknown tags. As as result, old UAs will ignore the 'STYLE' element, but its content will be treated as part of the document body, and rendered as such. During a transition phase, 'STYLE' element content may be hidden using SGML comments:
<STYLE><!--
H1 { color: green }
--></STYLE>
Since the 'STYLE' element is declared as "CDATA" in the DTD (as defined in [4]), conformant SGML parsers will not consider the above style sheet to be a comment that is to be removed.
To reduce the size of style sheets, one can group selectors in comma-separated lists:
H1, H2, H3 { font-family: helvetica }
Similarly, declarations can be grouped:
H1 {
font-weight: bold;
font-size: 12pt;
line-height: 14pt;
font-family: helvetica;
font-style: normal
}
In addition, some properties have their own grouping syntax:
H1 { font: bold 12pt/14pt helvetica }
which is equivalent to the previous example.
In the first example, the color of 'H1' elements was set to blue. Suppose we have an 'H1' element with an emphasized element inside:
<H1>The headline <EM>is</EM> important!</H1>
If no color has been assigned to 'EM', the emphasized "is" will inherit the color of the parent element, i.e. it will also appear in blue. Other style properties are likewise inherited, e.g. 'font-family' and 'font-size'.
Inheritance starts at the oldest ancestor, i.e. the top-level element. In HTML, this is the 'HTML' element which is followed by the 'BODY' element. In order to set a "default" style property, one can use 'BODY' as selector:
BODY {
color: black;
background: url(texture.gif) white;
}
This will work even if the author has omitted the 'BODY' tag (which is legal) since the parser will infer the missing tag. The example above sets the text color to be black and the background to be an image. The background will be white if the image is not available. (See the description of the 'background' property for more on this.)
Some style properties are not inherited from the parent element to the child element. Most often it is intuitive why this is not the case. E.g., the 'background' property does not inherit, but the parent element's background will shine through by default.
Often, the value of a property is a percentage that refers to another property:
P { font-size: 10pt }
P { line-height: 120% } /* relative to 'font-size', i.e. 12pt */
For each property that allows percentage values, it is defined what property it refers to. Children elements of 'P' will inherit the computed value of 'line-height' (i.e. 12pt), not the percentage.
To increase the granularity of control over elements, HTML3 [4][5] proposes a new attribute: 'CLASS'. All elements inside the 'BODY' element can be classed, and the class can be addressed in the style sheet:
<HTML>
<HEAD>
<TITLE>Title</TITLE>
<STYLE TYPE="text/css">
H1.punk { color: #00FF00 }
</STYLE>
</HEAD>
<BODY>
<H1 CLASS=punk>Way too green</H1>
</BODY>
</HTML>
The normal inheritance rules apply to classed elements; they inherit values from their ancestors in the document structure.
One can address all elements of the same class by omitting the tag name in the selector:
.punk { color: green } /* all elements with CLASS green */
Only one class can be specified per selector. 'P.punk.rap' is therefore an invalid selector in CSS1. (Contextual selectors, described below, can have one class per simple selector)
HTML3 also introduces the 'ID' attribute which is guaranteed to have a unique value over the document. It can therefore be of special importance as a style sheet selector, and can be addressed with a preceding '#':
#z098y { letter-spacing: 0.3em }
<P ID=z098y>Wide text</P>
Inheritance saves CSS designers typing. Instead of setting all style properties, one can create defaults and then list the exceptions. To give 'EM' elements within 'H1' a different color, one may specify:
H1 { color: blue }
EM { color: red }
When this style sheet is in effect, all emphasized sections, within or outside 'H1' will turn red. Probably, one only wanted 'EM' elements within 'H1' to turn red and this can be specified with:
H1 EM { color: red }
The selector is now a search pattern on the stack of open elements, and this type of selector is referred to as a "contextual selector". Contextual selectors consist of several simple selectors separated by white space (all selectors described up to now have been simple selectors). Only elements that match the last simple selector (in this case the 'EM' element) are addressed, and only so if the search pattern matches. Contextual selectors in CSS1 look for ancestor relationships, but other relationships (e.g. parent-child) may be introduced in later revisions. In the example above, the search pattern matches if 'EM' is a descendant of 'H1', i.e. if 'EM' is inside an 'H1' element.
UL LI { font-size: small }
UL UL LI { font-size: x-small }
Here, the first selector matches 'LI' elements with at least one 'UL' ancestor. The second selector matches a subset of the first, i.e. 'LI' elements with at least two 'UL' ancestors. The conflict is resolved by the second selector being more specific due to the longer search pattern. See the cascading order (section 3.2) for more on this.
Contextual selectors can look for tags, classes, IDs or combinations of these:
DIV P { font: small sans-serif }
.reddish H1 { color: red }
#x78y CODE { background: blue }
DIV.sidenote H1 { font-size: large }
This first selector matches all elements with class 'punk' that have an ancestor of element 'P' with class 'reddish'. The second selector matches all 'CODE' elements that are descendants of the element with 'ID=x78y'.
Several contextual selectors can be grouped together:
H1 B, H2 B, H1 EM, H2 EM { color: red }
Which is equivalent to:
H1 B { color: red }
H2 B { color: red }
H1 EM { color: red }
H2 EM { color: red }
Textual comments in CSS style sheets are similar to those in the C programming language:
EM { color: red } /* red, really red!! */
Comments cannot be nested.
In CSS1, style is normally attached to an element based on its position in the document structure. This simple model is sufficient for a wide variety of styles, but doesn't cover some common effects. The concept of pseudo-classes and pseudo-elements extend addressing in CSS1 to allow external information to influence the formatting process.
Pseudo-classes and pseudo-elements can be used in CSS selectors, but do not exist in the HTML source. Rather, they are "inserted" by the UA under certain conditions to be used for addressing in style sheets. They are referred to as "classes" and "elements" since this is a convenient way of describing their behavior. More specifically, their behavior is defined by a "fictional tag sequence" where they have a well-defined order.
Pseudo-elements are used to address sub-parts of elements, while pseudo-classes allow style sheets to differentiate between different element types.
User agents commonly display newly visited anchors differently from older ones. In CSS1, this is handled through pseudo-classes on the 'A' element:
A:link { color: red } /* unvisited link */
A:visited { color: blue } /* visited links */
A:active { color: orange } /* active links */
All 'A' elements with an 'HREF' attribute will be put into one of these groups (i.e. target anchors are not affected). UAs may choose to move an element from 'visited' to 'link' after a certain time.
The formatting of an anchor pseudo-class is exactly as if the class had been inserted manually. Anchor pseudo-classes on elements other than 'A' have no effects.
Pseudo-classes can be used in contextual selectors:
A:link IMG { border: solid blue }
Also, pseudo-classes can be combined with normal classes:
A.external:visited { color: blue }
<A CLASS=external HREF="http://out.side/">external link</A>
If the link in the above example has been visited, it will be rendered in blue. Note that normal class names precede pseudo-classes in the selector.
Some common typographic effects are associated not with structural elements but rather typographical items as formatted on the canvas. In CSS1, two such typographical items can be addressed through pseudo-elements: the first line of an element, and the first letter.
The 'first-line' pseudo-element is used to apply special styles to the first line as formatted on the canvas:
<STYLE TYPE="text/css">
P:first-line { font-style: small-caps }
</STYLE>
<P>The first line of an article in Newsweek.
On an text-based UA, this could be formatted as:
THE FIRST LINE OF AN article in Newsweek.
(In the above example, the UA chose to replace small-caps text with capital letters since small-caps fonts were not available. Note that this specification does not describe how UAs should render documents when the necessary resources, e.g. colors and fonts, are not available.)
The fictional tag sequence in the above example is: "<P> <P:first-line> The first line of an </P:first-line> article in Newsweek.</P>". The 'first-line' end tag is inserted at the end of the first line as formatted on the canvas.
The 'first-line' pseudo-element can only be attached to a block-level element.
The 'first-letter' pseudo-element is used for "initial caps" and "drop caps" which are common typographic effects. All normal properties 'first-letter' elements and the formatting should be accordingly. This is how you could make a dropcap initial letter span two lines:
<HTML>
<HEAD>
<TITLE>Title</TITLE>
<STYLE TYPE="text/css">
P { font-size: 12pt; line-height: 12pt; }
P:first-letter { font-size: 200%; float: left contour; }
SPAN { text-transform: uppercase; }
</STYLE
</HEAD>
<BODY>
<P><SPAN>The first</SPAN> few words of an article in The Economist.</P>
</BODY>
</HTML>
(The 'SPAN' element is being proposed as a new character-level element for HTML3)
If an text-based UA supports the 'first-letter' pseudo-element (we do not expect them to), the above could be formatted as:
___ | HE FIRST few words |of an article in the Economist..
The fictional tag sequence is: "<P> <SPAN> <P:first-letter> T </P:first-letter> he first </SPAN> few words of an article in the Economist. </P>". Note that the 'first-letter' pseudo-element tags abut the content (i.e. the initial character), while the 'first-line' pseudo-element start tag is inserted right after the start tag of the element they are attached to.
The UA defines what characters are inside the 'first-letter' element. Normally, quotes that precede the first letter should be included:
|| /\ is the first
/ \ letter of the
/----\ alphabet,"
/ \ said the
wizard to the child.
When the paragraph starts with other punctuation (e.g. parenthesis, ellipsis points) or other characters that are normally not considered letters (e.g. digits and mathematical symbols), 'first-letter' pseudo-elements are usually ignored.
Some languages may have specific rules about how to treat certain letter combinations. In Dutch, for example, if the letter combination "ij" appears in the beginning of a word, they should both be considered within the 'first-letter' pseudo-element.
The 'first-letter' pseudo-element can only be attached to a block-level element.
In a contextual selector, pseudo-elements are only allowed in the last simple selector:
BODY P:first-letter { color: purple }
Pseudo-elements can be combined with classes in selectors:
P.initial:first-letter { color: red }
<P CLASS=initial>First paragraph</A>
The above example would make the first letter of all 'P' elements with 'CLASS=initial' red. When combined with classes or pseudo-classes, pseudo-elements should be specified at the end of the selector. Only one pseudo-element can be specified per selector.
Several pseudo elements can be combined:
P { color: red; font-size: 12pt }
P:first-letter { color: green; font-size: 200% }
P:first-line { color: blue }
In this example, the first letter of each 'P' element would be green with a font size of 24pt. The rest of the first line (as formatted on the canvas) would be blue while the rest of the paragraph would be red. The fictional tag sequence is: "<P> <P:first-line> <P:first-letter> ... </P:first-letter> ... <P:/first-line> </P>".
Note that the 'first-letter' element is inside the 'first-line' element. Properties set on 'first-line' will be inherited by 'first-letter', but are overridden if the same property is set on 'first-letter'.
Possibly, pseudo-elements influencing the same content can be set on different elements:
BODY:first-letter { color: red }
P:first-letter { font-size: 200% }
<BODY><P>some text ...
The fictional tag sequence of the above example is "<BODY> <BODY:first-letter> <P> <P:first-letter> some text ..."
If a pseudo-element breaks up a real element the necessary extra tags must be regenerated in the fictional tag sequence. For example, if a SPAN element spans over a </P:first-line> tag, a set of SPAN end and start tags must be regenerated and the fictional tag sequence becomes "<P><P:first-line><SPAN>This text is inside a long</SPAN></P:first-line><SPAN>span element</SPAN>".
In CSS, more than one style sheet can influence the presentation simultaneously. There are two main reasons for this feature: modularity and author/reader balance.
@import url(http://www.style.org/punk);
@import url(http://www.style.org/funk);
H1 { color: red } /* override imported sheets */
Sometimes conflicts will arise between the style sheets that influence the presentation. Conflict resolution is based on each style rule having a weight. By default, the weights of the reader's rules are less than the weights of rules in the author's documents. I.e., if there are conflicts between the style sheets of an incoming document and the reader's personal sheets, the author's rules will be used. Both reader and author rules override UA's default values.
Style sheet designers can increase the weights of their rules:
H1 { color: red ! important }
P { font-size: 12pt ! important }
An important reader rule will override an author rule with normal weight. An important author rule will override an important reader rule.
Conflicting rules are intrinsic to the CSS mechanism. To find the value for an element/property combination, the following algorithm should be followed:
LI {...} /* a=0 b=0 c=1 -> specificity = 1 */
UL LI {...} /* a=0 b=0 c=2 -> specificity = 2 */
UL OL LI {...} /* a=0 b=0 c=3 -> specificity = 3 */
LI.red {...} /* a=0 b=1 c=1 -> specificity = 11 */
UL OL LI.red {...} /* a=0 b=1 c=3 -> specificity = 13 */
#x34y {...} /* a=1 b=0 c=0 -> specificity = 100 */
Pseudo-elements and pseudo-classes are counted as normal elements and classes, respectively.
The search for the property value can be terminated whenever one rule has a higher weight than the other rules that apply to the same element/property combination.
This strategy gives author's style sheets considerably higher weight than those of the reader. It is therefore important that the reader has the ability to turn off the influence of a certain style sheet, e.g. through a pull-down menu.
A 'STYLE' attribute on an element (see section 1.1 for an example) should be considered as if an ID attribute had been specified at the end of the style sheet.
The UA may choose to honor other stylistic attributes (e.g. 'ALIGN') as if a 'STYLE' attribute had been used. When in conflict with other stylistic attributes, the 'STYLE' attribute should win.
This document suggests a simple box-oriented formatting model. Each block-level element (e.g. 'H1' and 'P', but not 'EM') is surrounded by a box. The size of the box is the sum of the element width (i.e. formatted text or image), the padding, the border and the margins:
_______________________________________
| |
| margin (transparent) |
| _________________________________ |
| | | |
| | border | |
| | ___________________________ | |
| | | | | |
| | | padding | | |
| | | _____________________ | | |
| | | | | | | |
| | | | content | | | |
| | | |_____________________| | | |
| | |___________________________| | |
| |_________________________________| |
|_______________________________________|
| element width |
| box width |
The size of the margin, border and padding are set with the 'margin', 'border' and 'padding' properties respectively. The padding area uses the same background as the element itself (set with the 'background' property). The color and style for the border is set with the 'border' property. The margins are always transparent, so the parent element will shine through.
The following example shows how margins and padding format a 'UL' element with two children. To simplify the diagram there are no borders.
<STYLE TYPE="text/css">
UL {
background: red;
margin: A B C D; /* let's pretend we have constants in CSS1 */
padding: E F G H; /* " */
}
LI {
color: white;
background: blue; /* so text is white on blue */
margin: a b c d; /* let's pretend we have constants in CSS1 */
padding: e f g h; /* " */
}
</STYLE>
..
<UL>
<LI>1st element of list
<LI>2nd element of list
</UL>
_______________________________________________________ | | | A UL margin (transparent) | | _______________________________________________ | | D | | B | | | E UL padding (red) | | | | _______________________________________ | | | | H | | F | | | | | a LI margin (transparent, | | | | | | so red shines through) | | | | | | _______________________________ | | | | | | d | | b | | | | | | | e LI padding (blue) | | | | | | | | | | | | | | | | h 1st element of list f | | | | | | | | | | | | | | | | g | | | | | | | |_______________________________| | | | | | | | | | | | | max(a, c) | | | <- note the max | | | | | | | | | _______________________________ | | | | | | | | | | | | | | d | e LI padding (blue) | | | | | | | | | | | | | | | | h 2nd element of list f | | | | | | | | | | | | | | | | g | | | | | | | |_______________________________| | | | | | | | | | | | | c LI margin (transparent, | | | | | | so red shines through) | | | | | |_______________________________________| | | | | | | | | G | | | |_______________________________________________| | | | | C | |_______________________________________________________|
Technically, padding and margin properties are not inherited. But, as the example shows, the placement of an element is relative to ancestors and siblings so these elements' padding and margin properties have an effect on their children.
If the border width had been set (the default value is '0'), the border would have appeared between the padding and the margins.
The width of the margins specify the minimum distance to the edges of surrounding boxes. Two or more adjoining margins (i.e., with no border, padding or content between them) are collapsed to use the maximum of the margin values. In the example above, the margins between the two 'LI' elements are collapsed by using the maximum of the first LI element's 'margin-bottom' and the second LI element's 'margin-top'. Similarly, if the padding between the 'UL' and the first 'LI' element (the "E" constant) had been zero, the margins of the UL and first LI elements should have been collapsed.
In the case of negative margins, the absolute maximum of the negative adjoining margins should be deducted from the maximum of the positive adjoining margins.
Seven length units influence the horizontal dimension of a box: left margin, left border, left padding, width, right padding, right border, right margin. Added up, these have to be equal to the width of the parent element. Therefore, one cannot specify values for all properties and expect them to be honored. The relative strengths between them are as follows:
By default, the value of the 'width' property is 'auto' which means it will be automatically calculated based on the other properties' values. However, if 'width' is assigned another value, or the dimensions don't add up for other reasons, the property with the lowest rank will be assigned 'auto', i.e. automatically calculated.
Elements with a 'display' property value of 'list-item' are preceded by a label. The type of label is determined by the 'list-style' property. The label is not considered to be a part of the content, and will be placed outside the content. The rendering of the label should be based on the font and color properties of the element it belongs to.
The canvas is the part of the UA's drawing surface onto which documents are rendered. No structural element of a document correspond to the canvas, and this creates a problem when the document doesn't fill the whole canvas: how should the unfilled area be rendered? HTML extensions have set a precedence in this area: attributes on the 'BODY' element set the background of the whole canvas. To support designers' expectations, CSS1 introduces a special rule to find the canvas background:
If the 'background' value of the 'HTML' element is different from 'transparent' then use it, else use the 'background' value of the 'BODY' element. If the resulting value is 'transparent', the rendering is undefined.
This rule allows:
<HTML STYLE="background: url(http://style.com/marble.png)"> <BODY STYLE="background: red">
In the example above, the canvas will be covered with "marble". The background of the 'BODY' element (which may or may not fully cover the canvas will be red.
Until other means of addressing the canvas become available, we recommend setting canvas properties on the 'BODY' element.
Using the 'float' property, elements can be declared to be outside the normal flow of elements. For example, by setting the 'float' property of an image to 'left', the normal flow will wrap around on the right side. The image's position will be taken from the margin properties.
<STYLE>
IMG.icon {
float: left;
margin-left: 2em;
}
</STYLE>
<BODY>
<IMG CLASS=icon SRC=star.gif>
<P>Some text to show how text wraps around floating images
</BODY>
The above example could be formatted as:
___
| |Some text
| * |to show how
|___|text wraps
around floating images
By slightly modifying the 'float' declaration from the above example, an interesting visual effect can be achieved:
float: left contour;
Which could be formatted as:
Some text to show
| *how text wraps
around floating images
Since the image is transparent and the 'contour' value has been set, the text that normally wraps around the bounding box of the image now wraps around the contour of the image. What a contour is depends on the media type. If the floating element had been surrounded by a border or had a non-transparent background, then 'contour' would have no effect.
In both these examples, vertical bar and underscore characters are used to indicate the bounding box of the image.
Typically, only image elements are set to be floating. However, there is nothing that prevents the 'float' property to be used on other elements:
H1 {
font-size: medium;
float: left; /* run-in header */
}
A replaced element is an element which is replaced by content pointed to from the element. E.g., in HTML, the IMG element is replaced by the image pointed to by the SRC attribute. One can assume that replaced elements come with their own intrinsic width and height. If the CSS values for 'width' is 'auto', the intrinsic width should be used as the width of the element. If a value other than 'auto' is specified in the style sheet, this value should be used and the replaced element should be resized accordingly (the resize method will depend on the media type). The 'height' property is used in the same manner.
Style sheets influence the presentation of documents by assigning values to style properties. This section lists the defined style properties, and their corresponding list of possible values, of CSS1.
In the text below, the allowed values for each property are listed with a syntax like the following:
Value: N | NW | NE
Value: [ <length> | thick | thin ]{1,4}
Value: <url>? <color> [ / <color> ]?
Value: <url> || <color>
The words between "<" and ">" give a type of value. The most common types are <length>, <percentage>, <url>, <number> and <color>; these are described in the section on units. The more specialized types (e.g. <font-family> and <border-style>) are described under the property where they appear.
Other words are keywords that must appear literally. The slash (/) is also considered a keyword.
Several things juxtaposed mean that all of them must occur, in the given order. A bar (|) separates alternatives: one of them must occur. A double bar (A || B) means that either A or B or both must occur, in any order. Brackets ([]) are for grouping. Juxtaposition is stronger than the double bar, and the double bar is stronger than the bar. Thus "a b | c || d e" is equivalent to "[ a b ] | [ c || [ d e ]]".
Every type, keyword, or bracketed group may be followed by one of the following modifiers:
Setting font properties will be among the most common uses of style sheets. Unfortunately, there exists no well-defined and universally accepted taxonomy for classifying fonts, and terms that apply to one font family may not be appropriate for others. E.g. 'italic' is commonly used to label slanted text, but the term is not appropriate for sans-serif fonts (whose slanted fonts are called 'oblique'). This specification suggests a liberal terminology for describing fonts, and a level of detail similar to common desktop publishing applications.
Additional font characteristics are being worked on in the W3C Fonts working group. These will complement the properties defined in CSS1.
A font is assumed to come with a certain encoding, i.e., a table that maps characters to glyphs. CSS1 assumes that a suitable table exists; it has no provisions for changing that table. A "character" in this respect is an abstract notion, it is not the number or bit-pattern that is found in the document, nor is it the glyph that appears on the screen. It is something in between. In many documents the byte with number 65 will represent the character "A", which will then, subject to the character-to-glyph mapping, be represented as some "A"-like shape. In non-western languages the relation between bytes and the characters they represent is much more complex. So, CSS deals with characters, and whether a particular implementation represents an "A" internally by the number 65 or something completely different is of no concern here. The character-to-glyph table for each font has one entry for each possible character in an HTML document. Each entry is either empty, meaning that the font doesn't have a glyph for that character, or it points to a glyph.
Many fonts will only have glyphs for a few hundred characters. For example, fonts for western languages usually have some 200 glyphs. The fact that most encoding tables will be quite sparse has important consequences:
<EM STYLE="font-family: Symbol">abcd</EM>
The example above should have no visual effect whatsoever, since the four characters 'abcd' have no glyphs in the 'Symbol' font. Older browsers that work with 8-bit characters internally often fail to check the font encoding. Typically, the above fragment will show four Greek letters. Authors should not rely on this behavior.
Value: <absolute-size> | <relative-size> | <length> | <percentage>
Initial: medium
Applies to: all elements
Inherited: yes
Percentage values: relative to parent element's font size
If the value is a number, it is interpreted as a relative keyword where a value of '1' is equivalent to 'bigger'. For example, if the parent element has a font size of 'medium', a value of '-2' will make the font size of the current element be 'x-small'.
Length and percentage values should not take the font size table into account when calculating the font size of the element.
For most properties, length values refer to the font size of the current element. On the 'font-size' property' length units (e.g. 'em' and 'ex'), refer to the font size of the parent element.
Note that an application may reinterpret an explicit size, depending on the context. E.g., inside a VR scene a font may get a different size because of perspective distortion.
Examples:
P { font-size: 12pt; } /* absolute size: be careful! */
BLOCKQUOTE { font-size: -1 }
EM { font-size: +1 } /* the '+' is optional */
EM { font-size: 150% }
EM { font-size: 1.5em }
If the suggested scaling factor of 1.5 is used, the latter three rules are identical.
Value: [[<family-name> | <generic-family>],]* [<family-name> | <generic-family>]
Initial: UA specific
Applies to: all elements
Inherited: yes
Percentage values: N/A
The value is a prioritized list of font family names and/or generic family names. Unlike most other CSS1 properties, values are separated by a comma to indicate that they are alternatives:
BODY { font-family: gill, helvetica, sans-serif }
There are two types of list values:
BODY { font-family: "new century schoolbook", serif }
<BODY STYLE="font-family: 'My own font', fantasy">
If quoting is omitted, any white space characters before and after the font name is ignored and any sequence of white space characters inside the font name is converted to a single space.
Ideally, the style sheet designer should specify only one font, and the font manager should return the best alternative (perhaps by taking visual similarity, visual quality and performance into account). Unfortunately, current rendering environments do not offer this level of service, and it is beyond the style sheet mechanism to do so. Therefore, a prioritized list of alternative families can be supplied. This practice poses one problem: the UA must be able to determine if a font selection has been successful or not to know how far it should proceed in the list. One example: if the style sheet asks for 'univers' and the window system is smart enough to suggest 'helvetica' (which looks almost identical) as a replacement, is this a success or failure? This specification leaves the answer undefined for now.
Value: extra-light | light | demi-light | medium | demi-bold | bold | extra-bold | bolder | lighter
Initial: medium
Applies to: all elements
Inherited: yes
Percentage values: N/A
The keywords can either be absolute or relative:
P { font-weight: medium } /* absolute */
STRONG { font-weight: bolder } /* relative */
In the example above, 'STRONG' elements should be 'bolder' than their parent. If the parent has a 'font-weight' value 'medium', a 'STRONG' element will have the value 'bold'. I.e., a relative value indicates a change of two positions relative to the list of absolute values. Child elements inherit the resultant weight, not the keyword value.
If the desired font weight is not available, the UA selects an approximation strategy.
Value: normal | italic || small-caps | oblique || small-caps | small-caps
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: N/A
Legal combinations of the values are:
If the preferred font style cannot be accomplished, the UA should make best efforts to find acceptable substitutions. Often, an 'oblique' font can be substituted by an 'italic' font. If 'small-caps' are not available, capital letters of a smaller font size can be used to render small characters if the resolution of the output medium is appropriate for this.
H1, H2, H3 { font-style: small-caps italic }
H1 EM { font-style: italic }
In the example above, emphasized text within 'H1' will appear in normal lower-case italic.
Value: <number> | <length> | <percentage>
Initial: UA specific
Applies to: block-level elements
Inherited: yes
Percentage values: refers to the font size of the element itself
The property sets the the distance between two adjacent lines' baselines. It only applies to block-level elements.
When a numerical value is specified, the line height is given by the font size of the current element multiplied with the numerical value. This differs from a percentage value in the way it inherits: when a numerical value is specified, child elements will inherit the factor itself, not the resultant value (as is the case with percentage and other units).
Negative values are not allowed.
The three rules in the example below have the same resultant line height:
DIV { line-height: 1.2; font-size: 10pt } /* number */
DIV { line-height: 1.2em; font-size: 10pt } /* length */
DIV { line-height: 120%; font-size: 10pt } /* percentage */
It is suggested that UAs set the initial value to be a number in the range of 1.0 to 1.2.
Value: [<font-weight> || <font-style>]? <font-size> [ / <line-height> ]? <font-family>
Initial: not defined
Applies to: all elements
Inherited: yes
Percentage values: allowed on <font-size> and <line-height> only
This syntax of this property is based on a traditional typographic shorthand notation to set multiple properties related to fonts: 'font-style', 'font-weight', 'font-size', 'line-height' and 'font-family'. For a definition of allowed and initial values, see the previously defined properties. Setting the 'font' property is equivalent to including separate declarations at the same point in the style sheet. Properties left out of a 'font' specification (e.g., 'line-height' is optional) are set to their initial value.
P { font: bold 12pt/14pt sans-serif }
P { font: 80% sans-serif }
P { font: bold italic x-large/110% "new century schoolbook", serif }
In the second rule, the font size percentage value ('80%') refers to the font size of the parent element. In the last rule, the line height percentage refers to the font size of the element itself.
Although not in accordance with the traditional notation, the font weight and font style must precede the font size to avoid ambiguity.
Value: <color>
Initial: UA specific
Applies to: all elements
Inherited: yes
Percentage values: N/A
This property describes the text color of an element, i.e. the "foreground" color. There are different ways to specify red:
EM { color: red } /* natural language */
EM { color: rgb(255,0,0) } /* RGB rage 0-255 */
See section 6.3 for a complete description of possible color values.
Value: transparent | <color> || <url> || <repeat> || <scroll> || <position>
Initial: transparent
Applies to: all elements
Inherited: no
Percentage values: N/A
This property describes the background of an element, i.e. the surface onto which the content (e.g. text) is rendered. Values on this property can be one of:
P { background: transparent } /* transparent */
BODY { background: red } /* one color */
BODY { background: url(chess.png) 50% repeat fixed } /* img + controls */
This property does not inherit, but the parent element's background will shine through by default due to the initial transparent value. If neither an image or a color is found, a value of 'transparent' is assumed.
If an image is found through the URL, it will be overlaid on top of any color specified. The color is used:
BODY { background: url(marble.png) repeat-x }
In the example above, the image will only be repeated horizontally. Similarly, a value of 'repeat-y' will repeat the image vertically.
With a value pair of '0% 0%', the upper left corner of the image is placed in the upper left corner of the box that surrounds the content of the element (i.e., not the box that surrounds the padding, border or margin). A value pair of '100% 100%' places the lower right corner of the image in the lower right corner of the element. With a value pair of '14% 84%', the point 14% across and 84% down the image is to be placed at the point 14% across and 84% down the element.
If only one value is given, it sets both the horizontal and vertical offset of the background image. If two values are given, the horizontal position comes first.
Negative positions are allowed.
If the background image is fixed with regard to the canvas (see the <scroll> value above), the image is placed relative to the canvas instead of the element. E.g.:
BODY {
background: url(logo.png) fixed 100% 100%;
}
In the example above, the image is placed in the lower right corner of the canvas.
The keyword values are defined as follows:
left = 0% center = 50% right = 100% top = 0% middle = 50% bottom = 100%
Value: normal | <length>
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: N/A
The length unit indicates an addition to the default space between words. Values can be negative, but there may be implementation-specific limits. The UA is free to select the exact spacing algorithm. The word spacing may also be influenced by justification (which is a value of the 'align' property).
H1 { word-spacing: 1em }
Here, the word-spacing between each word in 'H1' elements would be increased with '1em'.
Value: normal | <length>
Initial: normal
Applies to: all elements
Inherited: yes
Percentage values: N/A
The length unit indicates an addition to the default space between characters. Values can be negative, but there may be implementation-specific limits. The UA is free to select the exact spacing algorithm. The letter spacing may also be influenced by justification (which is a value of the 'align' property).
BLOCKQUOTE { letter-spacing: 0.1em }
Here, the word-spacing between each character in 'BLOCKQUOTE' elements would be increased with '0.1em'.
Value: none | [ underline | overline | line-through | blink ]+
Initial: none
Applies to: all elements
Inherited: no, but see clarification below
Percentage values: N/A
This property describes decorations that are added to the text of an element. If the element has no text (e.g. the IMG element in HTML) or is an empty element (e.g. "<EM></EM>"), this property has no effect.
The color(s) required for the text decoration should be derived from the 'color' property value.
This property is not inherited, but children elements should match their ancestor. E.g., if an element is underlined, the line should span the child elements. The color of the underlining will remain the same even if descendant elements have different 'color' values.
A:link, A:visited, A:active { text-decoration: underline }
The example above would underline the text of all active links.
We expect UA vendors to propose several new values on this property. Formatters should treat unknown values as 'underline'.
Value: baseline | sub | super | top | text-top | middle | bottom | text-bottom | <percentage>
Initial: baseline
Applies to: all elements
Inherited: yes
Percentage values: refer to the 'line-height' of the element itself
The property affects the vertical positioning of the element. One set of keywords is relative to the parent element:
Using the 'top' and 'bottom' alignment, unsolvable situations can occur where element dependencies form a loop.
Percentage values refer to the 'line-height' of the element itself. E.g., a value of '-100%' will lower the element to where the baseline of the next line should have been.
Value: capitalize | uppercase | lowercase | none
Initial: none
Applies to: all elements
Inherited: yes
Percentage values: N/A
H1 { text-transform: uppercase }
The example above would put 'H1' elements in uppercase text.
Value: left | right | center | justify
Initial: UA specific
Applies to: block-level elements
Inherited: yes
Percentage values: N/A
This property describes how text is aligned within the element. The actual justification algorithm used is UA and human language dependent.
Example:
DIV.center { text-align: center }
Since this property inherit, all block-level elements inside the 'DIV' element with 'CLASS=center' will be centered. Note that alignments are relative to the width of the element, not the canvas. If 'justify' is not supported, the UA will supply a replacement. Typically, this will be 'left' for western languages.
Value: <length> | <percentage>
Initial: 0
Applies to: block-level elements
Inherited: yes
Percentage values: refer to parent element's width
The property specifies indent that appears before the first formatted line. 'text-indent' may be negative, but there may be implementation-specific limits. An indent is not inserted in the middle of an element that was broken by another (such as 'BR' in HTML).
Example:
P { text-indent: 3em }
See the formatting model (section 4) for examples on how to use the box properties.
Value: [ <length> | <percentage> | auto ]{1,4} (for 'margin' property)
Initial: 0
Applies to: all elements
Inherited: no
Percentage values: refer to parent element's width
These properties set the margin of an element: the 'margin' property sets the border for all four sides while the other properties only set their respective side.
For the 'margin' property, the four lengths apply to top, right, bottom and left respectively. If there is only one value, it applies to all sides, if there are two or three, the missing values are taken from the opposite side.
BODY { margin: 2em } /* all margins set to 2em */
BODY { margin: 1em 2em } /* top & bottom = 1em, right & left = 2em */
BODY { margin: 1em 2em 3em } /* top=1em, right=2em, bottom=3em, left=2em */
The 'margin' property is shorthand for setting 'margin-top', 'margin-right' 'margin-bottom' and 'margin-left' at the same place in the style sheet. These properties allow only one value. The last rule of the example above is equivalent to the example below:
BODY {
margin-top: 1em;
margin-right: 2em;
margin-bottom: 3em;
margin-left: 2em; /* copied from opposite side (right) */
}
The margins express the minimal distance between the borders of two adjacent elements. See the formatting model (section 4) for an example.
When margin properties are applied to replaced elements (e.g. IMG), they express the minimal distance from the replaced element to any of the content of the parent element.
Negative margin values are allowed, but there may be implementation-specific limits.
Value: [ <length> | <percentage> | auto ]{1,4}
Initial: 0
Applies to: all elements
Inherited: no
Percentage values: refer to parent element's width
The property describes how much space to insert between the border and the content (e.g. text or image). The order is top, right, bottom, left. If there is only one value, it applies to all sides, if there are two or three, the missing values are taken from the opposite side.
The surface of the padding area is set with the 'background' property.
H1 { background: white; padding: 1em }
The example above sets a 1em padding on all sides. The 'em' unit is relative to the element's font.
Padding values cannot be negative. See the formatting model (section 4) for more on these properties.
Value: <border-width> || <border-style> || <url> || <color>
Initial: medium none
Applies to: all elements
Inherited: no
Percentage values: N/A
These properties set the border of an element: the 'border' property sets the border for all four sides while the other properties only set their respective side.
The border is drawn in the image pointed to by the URL. If no URL is specified, or when the image is not available, the color value is used.
If an image is found through the URL, it will be used as a texture (i.e. repeated throughout the border) to draw the border. The <color> value, if specified, is used:
P {
color: black;
background: white;
border: solid;
}
In the above example, the border will be a solid black line.
The keyword widths are constant throughout a document:
H1 { border: solid thick red }
P { border: solid thick blue }
In the example above, 'H1' and 'P' elements will have the same border width regardless of font size. To do relative width, the 'em' unit can be used:
H1 { border: solid 0.5em }
The border styles mean:
If the element is 'inline' and spans several lines, the UA is free to render one border on each line, possibly with omitted edges.
Value: <length> | <percentage> | auto
Initial: auto
Applies to: all elements
Inherited: no
Percentage values: refer to parent element's width
This property can be applied to text elements, but it is most useful with inline images and similar insertions. The width is to be enforced by scaling the image if necessary. When scaling, the aspect ratio of the image should be preserved if the 'height' property is 'auto'.
Example:
IMG.icon { width: 100px }
See the formatting model (section 4) for a description of the relationship between this property and the margin and padding.
Value: <length> | auto
Initial: auto
Applies to: block-level and replaced elements
Inherited: no
Percentage values: N/A
This property can be applied to text, but it is most useful with inline images and similar insertions. The height is to be enforced by scaling the image if necessary. When scaling, the aspect ratio of the image should be preserved if the 'width' property is 'auto'.
Example:
IMG.icon { height: 100px }
If applied to a textual element, the height can be enforced with e.g. a scrollbar.
Value: <side-flow> || <wrap>
Initial: none box
Applies to: all elements
Inherited: no
Percentage values: N/A
Using the 'float' property, one can set an element to be floating and indicate how other content will wrap around it.
This property is most often used with inline images.
Value: none | left | right | both
Initial: none
Applies to: all elements
Inherited: no
Percentage values: N/A
This property specifies if an element allows floating elements on its sides. More specifically, the value of this property lists the sides where floating elements are not accepted. With 'clear' set to 'left', an element will be moved below any floating element on the left side. With 'clear' set to 'none', floating elements are allowed on all sides. Example:
H1 { clear: left }
These properties classify elements into categories more than they set specific visual parameters.
Value: block | inline | list-item | none
Initial: according to HTML
Applies to: all elements
Inherited: no
Percentage values: N/A
This property indicates if an element is inline (e.g. 'EM' in HTML), block-level (e.g. 'H1' in HTML), or a block-level list item (e.g. 'LI' in HTML). For HTML documents, the initial value will be taken from the HTML specification.
A value of 'none' turns the display of the element, including children elements and the surrounding box, off.
P { display: block }
EM { display: inline }
LI { display: list-item }
IMG { display: none }
The last rule turns the display of images off.
Note that HTML defines what elements are block-level (called "Block Structuring Elements" in [2]) and inline (called "Phrase Markup" in [2]), and this may be hardcoded into some UA implementations.
Value: <keyword> || <position> || <url>
Initial: disc outside
Applies to: elements with 'display' property value 'list-item'
Inherited: yes
Percentage values: N/A
The 'list-style' property describes how list items (i.e. elements with a 'display' value of 'list-item') are formatted.
This property can be set on any element, and it will inherit normally down the tree. However, the 'list-style' will only be displayed on elements with a 'display' value of 'list-item'. In HTML this is typically the case for the 'LI' element.
UL { list-style: disc inside }
UL UL { list-style: circle outside }
LI.square { list-style: square }
OL { list-style: decimal } /* 1 2 3 4 5 etc. */
OL { list-style: lower-roman } /* a b c d e etc. */
A URL value can be combined with any other value:
UL { list-style: url(http://png.com/ellipse.png) disc }
In the example above, the 'disc' will be used when the image is unavailable.
An example of an 'outside' rendering:
* first list item
comes first
* second list item
comes second
The same example formatted with 'inside':
* first list item comes first * second list item comes second
Value: normal | pre | nowrap
Initial: according to HTML
Applies to: block-level elements
Inherited: yes
Percentage values: N/A
Declares how white space inside the element should be handled: the 'normal' way (where white space is collapsed), as 'pre' (which behaves like the 'PRE' element in HTML) or as 'nowrap' (where wrapping is done only through BR elements):
PRE { white-space: pre } /* initial value */
The format of a length value is an optional sign character ('+' or '-', with '+' being the default) immediately followed by a number (with or without a decimal point) immediately followed by a unit identifier (a two-letter abbreviation).
Some properties allow negative length units, but this may complicate the formatting model and there may be implementation-specific limits. If a negative length value cannot be supported, it should be clipped to the nearest value that can be supported.
There are three types of length units: relative, pixel and absolute. Relative units specify a length relative to another length property. Style sheets that use relative units will more easily scale from one medium to another (e.g. from a computer display to a laser printer). Percentage units (described below) and keyword values (e.g. 'x-large') offer similar advantages.
Child elements inherit the computed value, not the relative value:
BODY {
font-size: 12pt;
text-indent: 3em; /* i.e. 36pt */
}
H1 { font-size: 15pt }
In the example above, the 'text-indent' value of 'H1' elements will be 36pt, not 45pt.
These relative units are supported:
H1 { margin: 0.5em } /* ems, the height of the element's font */
H1 { margin: 1ex } /* x-height, ~ the height of the letter 'x' */
P { font-size: 12px } /* pixels, relative to canvas */
Pixel units, as used in the last rule, are relative to the resolution of the canvas, i.e. most often a computer display. If the pixel density of the output device is very different from that of a typical computer display, the UA should rescale pixel values. The suggested "reference pixel" is the visual angle of one pixel on a device with a pixel density of 90dpi and a distance from the reader of an arm's length.
Absolute length units are only useful when the physical properties of the output medium is known. These absolute units are supported:
H1 { margin: 0.5in } /* inches, 1in = 2.54cm */
H2 { line-height: 3cm } /* centimeters */
H3 { word-spacing: 4mm } /* millimeters */
H4 { font-size: 12pt } /* points, 1pt = 1/72 in */
H4 { font-size: 1pc } /* picas, 1pc = 12pt */
The format of a length value is a number (with or without a decimal point) immediately followed by '%'. Percentage values are always relative to a length unit. Each property that allows percentage units also define what length unit they refer to. Most often this is the font size of the element itself:
P { line-height: 120% } /* 120% of the element's 'font-size' */
Child elements inherit the resultant value, not the percentage value.
A color is a either a color name or a numerical RGB specification.
The suggested list of color names is: aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, purple, red, silver, teal, white, and yellow. These 16 colors are taken from the Windows VGA palette and will also be used in forthcoming HTML 3.2. The RGB values for these color name are not defined in this specification.
BODY {color: black; background: white }
H1 { color: maroon }
H2 { color: olive }
RGB colors are specified in the sRGB color space as defined in the appendix of [6]. UAs should make reasonable efforts to render colors accurately according to the sRGB specification.
There are different ways to specify red:
EM { color: #f00 } /* #rgb */
EM { color: #ff0000 } /* #rrggbb */
EM { color: rgb(255,0,0) } /* integer range 0 - 255 */
EM { color: rgb(255,0,0) } /* integer range 0 - 255 */
EM { color: rgb(100%, 0%, 0%) } /* float range 0.0% - 100.0% */
Note that the three-digit RGB notation (#rgb) is converted into six-digit form (#rrggbb) by replicating digits, not by adding zeros. For example, #fb0 expands to #ffbb00.
Values outside the numerical ranges should be clipped. The three rules below are therefore equivalent:
EM { color: rgb(255,0,0) } /* integer range 0 - 255 */
EM { color: rgb(300,0,0) } /* clipped to 255 */
EM { color: rgb(110%, 0%, 0%) } /* clipped to 100% */
A Uniform Resource Locator (URL) is identified with a functional notation:
BODY { background: url(http://www.bg.com/pinkish.gif) }
Partial URLs are interpreted relative to the source of the style sheet, not relative to the document:
BODY { background: url(yellow) }
W3C will publish a test suite of HTML documents with CSS1 style sheets that conforming UAs must format correctly. A UA conforms to the CSS1 specification if it passes the test suite. The test suite will ensure that a conforming UA:
Example of constraints of the presentation medium are: limited resources (fonts, color) and limited resolution (so margins may not be accurate). In these cases, the UA should approximate the style sheet values. Also, different user interface paradigms may have their own constraints: a VR browser may rescale the document based on its "distance" from the user.
UAs may offer readers the ability to set additional constraints on the presentation medium, e.g., to deal with visual impairments or to disable blinking.
Note that CSS1 does not specify all aspects of formatting. E.g., the UA is free to select a letter-spacing algorithm.
This specification also recommends, but doesn't require, that a UA:
The above conformance rules only describe functionality, not user interface.
This section and the next describe a "meta-syntax", in the sense that they define the limits within which CSS1 and all future extensions will have to remain. In other words, the CSS1 language is a subset of the language described here, and all later versions of CSS are intended to be subsets of that language as well. Implementors should create parsers that are "forward compatible": when unknown constructions are encountered, the text below tells the parser how much of the input can be safely skipped.
Here is an EBNF-like grammar:
stylesheet : CDO? rules? CDC?
rules : [ at-rule | rule ]+
rule : selector decl-block
at-rule : AT_KEYWORD token* [ ';' | decl-block | rule-block ]
selector : token+
decl-block : '{' declarations '}'
rule-block : '{' rules '}'
declarations : declaration? [ ';' declaration? ]*
declaration : IDENT ':' token+
token : IDENT | '[' token* ']' | '(' token* ')' |
SYMBOL | ':' | ',' | STRING | NUMBER | DIM
Note that this grammar is not LL(1); in practice, it will depend on the AT_KEYWORD whether an at-rule ends with a semicolon, a decl-block or a rule-block.
In English, the grammar says: a style sheet is a list of rules (normal rules or @-rules), each rule ends either with a semicolon (;) or with a block ({...}). Blocks contain either declarations (keyword-colon-value), or they contain more rules, in which case they may be nested. Note that nested blocks are not used in CSS1.
The style sheet may be enclosed in <!-- and -->, which is useful when it is embedded in an HTML document.
Besides the 9 punctuation characters ( , : ; [ ] ( ) { } ), the lexical tokens of CSS are:
IDENT : namestart namechar*
AT_KEYWORD : '@' IDENT
NUMBER : [ '-' | '+' ]? decimal+
| [ '-' | '+' ]? decimal* '.' decimal+
DIM : NUMBER IDENT
STRING : '"' [ stringchar | escape | sq ]* '"'
| sq [ stringchar | escape | '"' ]* sq
WS : whitespace+
CDO : '<' '!' '-' '-'
CDC : '-' '-' '>'
SYMBOL : symchar+
Using the following auxiliary definitions:
symchar : '`' | '~' | '!' | '@' | '#' | '$' | '%' | '^' | '&' |
'*' | '-' | '_' | '+' | '=' | '|' | '<' | '>' | '.' |
'/' | '?'
stringchar : <any printable character except ' and " including space>
decimal : '0' | ... | '9'
octal : '0' | ... | '7'
sq : <a single quote: ' >
namestart : letter | latin1 | escape
namechar : letter | latin1 | escape | decimal | '-'
letter : 'A' | ... | 'Z' | 'a' | ... | 'z'
latin1 : <a Latin 1 character (codes 161-255)>
escape : unicode | '\' printable
unicode : '\' octal [ octal [ octal [ octal [ octal octal? ]? ]? ]? ]?
printable : <any printable character including space>
whitespace : <tab> | <space> | <newline> | comment
comment : '/' '*' any* '*' '/'
any : <any character>
These definitions assume the Latin 1 character set. Future versions of CSS may widen some of them to allow Unicode characters.
Paraphrased in English:
In case of ambiguity, take the longest match, except for comments.
In order for CSS implementations to be interoperable, as well as being prepared for future extensions to CSS, a conforming UA should follow these parsing conventions:
Note: The CLASS attribute of HTML allows more characters in a class name than the set allowed for selectors above. In CSS level 1, these characters have to be escaped or written as Unicode numbers: "B&W?" can be written as "B\&W\?" or "B\46W\77", "È¿ÅÁ¿Â" (Greek: "kouros") has to be written as "\1710\1677\1705\1701\1677\1702". It is expected that in later versions of CSS more characters can be entered directly.
As a help to implementers, a more detailed grammar for CSS1 is included in appendix B.
[1] The W3C resource page on web style sheets (http://www.w3.org/pub/WWW/Style)
[2] RFC 1866: Hypertext Markup Language - 2.0, (ftp://ds.internic.net/rfc/rfc1866.txt). The specification is also available in hypertext form (http://www.w3.org/pub/WWW/MarkUp/html-spec/html-spec_toc.html)
[3] ISO 8879. Information Processing - Text and Office Systems - Standard Generalized Markup Language (SGML), 1986.
[4] HTML3 and Style Sheets (http://www.w3.org/pub/WWW/TR/WD-style.html)
[5] HyperText Markup Language Specification Version 3.0 (http://www.w3.org/pub/WWW/MarkUp/html3/CoverPage.html) The draft has now expired
[6] M Anderson, R Motta, S Chandrasekar, M Stokes: Proposal for a Standard Color Space for the Internet - sRGB (http://www.w3.org/pub/WWW/Printing/motta/W3Color.html)
[7] T Berk, L Brownston, A Kaufman: A New Color-Naming System for Graphics Languages, IEEE CG&A, May 1982
[8] DSSSL (http://occam.sjf.novell.com:8080/dsssl/dsssl96) is a tree transformation and style language from the SGML community. Bert Bos has written some notes on Translating CSS to DSSSL (http://www.w3.org/pub/WWW/Style/css/css2dsssl.html)
[9] Frame-based layout via Style Sheets (http://www.w3.org/pub/WWW/TR/WD-layout.html)
[10] PNG (Portable Network Graphics) Specification (http://www.w3.org/pub/WWW/TR/WD-png.html)
During the short life of HTML, there have been several style sheet proposals to which this proposal is indebted. Especially the proposals from Robert Raisch, Joe English and Pei Wei were influential.
Through "www-style@w3.org" and other electronic media, these people have contributed: Kevin Hughes, William Perry, Benjamin Sittler, Tony Jebson, Paul Prescod, Evan Kirshenbaum, Scott Bigham, Glenn Adams, Scott Preece, Greg Watkins, Jon Smirl, Jenny Raggett, Michael Seaton, Charles Peyton Taylor, Lauren Wood, Wayne Gramlich and Murray Altheim.
Also, thanks to Tim Berners-Lee, Vincent Quint, Cécile Roisin, Irčne Vatton, Phill Hallam-Baker, Yves Lafon, Yves Bertot, Colas Nahaboo, Philippe Kaplan, Gilles Kahn, Terry Allen, Daniel Connolly, Roy Fielding, Henrik Frystyk Nielsen, James Clark, David Siegel, Thomas Reardon, Chris Wilson, Lydja Williams, Mandira Virmani, Scott Isaacs, Robert Cailliau, Stephen Zilles, Daniel Veillard, Lou Montulli and Phil Karlton for constructive discussions.
Three people deserve special mentioning: Dave Raggett (for his encouragement and work on HTML3), Chris Lilley (for his continued contributions, especially in the area of colors) and Steven Pemberton (for his organizational as well as creative skills).
The following style sheet is written according to the suggested rendering in the HTML 2.0 specification. Some styles, e.g. colors, have been added for completeness. It is suggested that a style sheet similar to the one below is used as a UA default.
BODY {
margin: 1em;
font-family: serif;
line-height: 1.1;
background: white;
color: black;
border: black; /* used by the HR element */
}
H1, H2, H3, H4, H5, H6, P, UL, OL, DIR, MENU, DIV,
DT, DD, ADDRESS, BLOCKQUOTE, PRE, BR, HR { display: block }
B, STRONG, I, EM, CITE, VAR, TT, CODE, KBD, SAMP,
IMG, SPAN { display: inline }
LI { display: list-item }
H1, H2, H3, H4 { margin-top: 1em; margin-bottom: 1em }
H5, H6 { margin-top: 1em }
H1 { text-align: center }
H1, H2, H4, H6 { font-weight: bold }
H3, H5 { font-style: italic }
H1 { font-size: xx-large }
H2 { font-size: x-large }
H3 { font-size: large }
B, STRONG { font-weight: bolder } /* relative to the parent */
I, CITE, EM, VAR, ADDRESS, BLOCKQUOTE { font-style: italic }
PRE, TT, CODE, KBD, SAMP { font-family: monospace }
PRE { white-space: pre }
ADDRESS { margin-left: 3em }
BLOCKQUOTE { margin-left: 3em; margin-right: 3em }
UL, DIR { list-style: disc }
OL { list-style: decimal }
MENU { margin: 0 0 } /* tight formatting */
LI { margin-left: 3em }
DT { margin-bottom: 0 }
DD { margin-top: 0; margin-left: 5em }
HR { border-top: solid } /* 'border-bottom' could also have been used */
A:link { color: blue } /* unvisited link */
A:visited { color: red } /* visited links */
A:active { color: orange } /* active links */
/* setting the anchor border around IMG elements
requires contextual selectors */
A:link IMG { border: 2px solid blue }
A:visited IMG { border: 2px solid red }
A:active IMG { border: 2px solid orange }
The minimal CSS grammar that all implementations need to support is defined in section 7. However, that grammar is not very helpful when implementing CSS1. Hopefully, the yacc/lex grammar below better serves that purpose.
The format of the productions is optimized for human consumption and some shorthand notation beyond yacc is used:
* : 0 or more + : 1 or more ? : 0 or 1 | : separates alternatives [] : grouping
The productions are:
stylesheet
: CDO? import* rule* CDC?
;
import
: IMPORT_SYM [STRING|URL] ';' /* E.g., @import url(fun.css); */
;
unary_operator
: '-' | '+'
;
/*
* The only operators in level 1 are slash, space and comma.
* An expression `a b c, d e f' stands for a list
* [[a,b,c],[d,e,f]]. Note that `a,b,c' is the list
* [a,b,c], *not* [[a],[b],[c]].
*/
operator
: '/' | ',' | /* empty */
;
property
: IDENT
;
rule
: selector [ ',' selector ]*
'{' declaration [ ';' declaration ]* '}'
;
selector
: simple_selector+ [ ':' pseudo_element ]?
;
/*
* A simple_selector is something like H1, PRE.FOO,
* .FOO, etc., or it is an ID: #p004
*
* DOT_WO_WS is a `.' without preceding whitespace.
* DOT_W_WS is a `.' with preceding whitespace.
*/
simple_selector
: element_name pseudo_class? [ DOT_WO_WS class ]?
| DOT_W_WS class
| id_selector
;
element_name
: IDENT
;
pseudo_class
: LINK_PSCLASS
| VISITED_PSCLASS
| ACTIVE_PSCLASS
;
class
: IDENT
;
pseudo_element
: IDENT
;
id_selector
: '#' IDENT
;
declaration
: property ':' expr prio?
| /* empty */ /* Prevents syntax errors... */
;
prio
: IMPORTANT_SYM /* !important" */
;
expr
: term [ operator term ]*
;
term
: unary_operator?
[ NUMBER | STRING | PERCENTAGE | LENGTH | EMS | EXS
| IDENT | HEXCOLOR | URL | RGB ]
;
The following is the input to a lex/flex scanner. Note that this assumes an 8-bit implementation of (f)lex.
%{
#include "constants.h"
/*
The constants include definitions similar to the following:
#define INCH (25.4 * MM)
#define CM (10 * MM)
#define MM 1
#define PICA (12 * INCH/72 * MM)
#define POINT (INCH/72 * MM)
*/
%}
%a 3000
%o 4000
unicode \\[0-7]{1,6}
latin1 [ˇ-˙]
escape {unicode}|\\[ -~ˇ-˙]
stringchar {escape}|{latin1}|[ !#$%&(-~]
nmstrt [a-zA-Z]|{latin1}|{escape}
nmchar [-a-zA-Z0-9]|{latin1}|{escape}
ident {nmstrt}{nmchar}*
d [0-9]
notnm [^-a-zA-Z0-9\\]|{latin1}
w [ \t\n]*
num {d}+|{d}*\.{d}+
h [0-9a-fA-F]
h3 {h}{h}{h}
h6 {h3}{h3}
h9 {h3}{h3}{h3}
%x COMMENT
%%
"/*" {BEGIN(COMMENT);}
<COMMENT>"*/" {BEGIN(INITIAL);}
<COMMENT>\n {/* ignore */}
<COMMENT>. {/* ignore */}
@import return IMPORT_SYM;
"!"{w}important return IMPORTANT_SYM;
{ident} {yylval.sym = str2Symbol(yytext); return IDENT;}
\"({stringchar}|\')*\" |
\'({stringchar}|\")*\' {yylval.str = noquotes(yytext); return STRING;}
{num} {yylval.num = atof(yytext); return NUMBER;}
{num}"%" {yylval.num = atof(yytext); return PERCENTAGE;}
{num}pt/{notnm} {yylval.num = atof(yytext) * POINT; return LENGTH;}
{num}mm/{notnm} {yylval.num = atof(yytext); return LENGTH;}
{num}cm/{notnm} {yylval.num = atof(yytext) * CM; return LENGTH;}
{num}pc/{notnm} {yylval.num = atof(yytext) * PICA; return LENGTH;}
{num}in/{notnm} {yylval.num = atof(yytext) * INCH; return LENGTH;}
{num}px/{notnm} {yylval.num = atof(yytext) * pixelwd; return LENGTH;}
{num}em/{notnm} {yylval.num = atof(yytext); return EMS;}
{num}ex/{notnm} {yylval.num = atof(yytext); return EXS;}
":"link {return LINK_PSCLASS;}
":"visited {return VISITED_PSCLASS;}
":"active {return ACTIVE_PSCLASS;}
"#"{h9} |
"#"{h6} |
"#"{h3} {yylval.str = yytext; return HEXCOLOR;}
"url("[^\n)]+")" {yylval.str = noquotes(yytext+3); return URL;}
"rgb("{w}{num}%?{w},{w}{num}%?{w},{w}{num}%?"{w})" {yylval.str = yytext; return RGB;}
":" return ':';
^"." |
[ \t]+"." return DOT_W_WS;
"." return DOT_WO_WS;
"/" return '/';
"+" return '+';
"-" return '-';
"{" return '{';
"}" return '}';
";" return ';';
"," return ',';
"#" return '#';
[ \t]+ {/* ignore whitespace */}
\n {/* ignore whitespace */}
\<\!\-\- return CDO;
\-\-\> return CDC;
. {yyerror("Illegal character");}
The goal of the work on CSS1 has been to create a simple style sheet mechanism for HTML documents. The current specification is a balance between the simplicity needed to realize style sheets on the web, and pressure from authors for richer visual control. CSS1 offers:
<H1>H<FONT SIZE=-1>EADLINE</FONT></H1>
with the style sheet:
H1 { font-style: small-caps }
<H1>Headline</H1>
CSS1 does not offer:
We expect to see extensions of CSS in several directions:
We do not expect CSS to evolve into: