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2 Introduction to CSS2

Contents

2.1 A brief CSS2 tutorial for HTML

In this tutorial, we show how easy it can be to design simple style sheets. For this tutorial, you will need to know a little HTML (see [HTML40]) and some basic desktop publishing terminology.

We begin with a small HTML document:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
  <HEAD>
  <TITLE>Bach's home page</TITLE>
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

To set the text color of the H1 elements to blue, you can write the following CSS rule:

  H1 { color: blue }

A CSS 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 tries to influence only 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 HTML 4.0 specification defines how style sheet rules may be specified for HTML documents: either within the HTML document, or via an external style sheet. To put the style sheet into the document, use the STYLE element:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
  <HEAD>
  <TITLE>Bach's home page</TITLE>
  <STYLE type="text/css">
    H1 { color: blue }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

For maximum flexibility, we recommend that authors specify external style sheets; they may be changed without modifying the source HTML document, and they may be shared among several documents. To link to an external style sheet, you can use the LINK element:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
  <HEAD>
  <TITLE>Bach's home page</TITLE>
  <LINK rel="stylesheet" href="bach.css" type="text/css">
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

The LINK element specifies:

To show the close relationship between a style sheet and the structured markup, we continue to use the STYLE element in this tutorial. Let's add more colors:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
  <HEAD>
  <TITLE>Bach's home page</TITLE>
  <STYLE type="text/css">
    BODY { color: red }
    H1 { color: blue }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

The style sheet now contains two rules: the first one sets the color of the BODY element to 'red', while the second one sets the color of the H1 element to 'blue'. Since no color value has been specified for the P element, it will inherit the color from its parent element, namely BODY. The H1 element is also a child element of BODY but the second rule overrides the inherited value. In CSS there are often such conflicts between different values, and this specification describes how to resolve them.

CSS2 has more than 100 different properties, including 'color'. Let's look at some of the others:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<HTML>
  <HEAD>
  <TITLE>Bach's home page</TITLE>
  <STYLE type="text/css">
    BODY { 
      font-family: "Gill Sans", sans-serif;
      font-size: 12pt;
      margin: 3em; 
    }
  </STYLE>
  </HEAD>
  <BODY>
    <H1>Bach's home page</H1>
    <P>Johann Sebastian Bach was a prolific composer.
  </BODY>
</HTML>

The first thing to notice is that several declarations are grouped within a block enclosed by curly braces ({...}), and separated by semicolons, though the last declaration may also be followed by a semicolon.

The first declaration on the BODY element sets the font family to "Gill Sans". If that font isn't available, the user agent (often referred to as a "browser") will use the 'sans-serif' font family which is one of five generic font families which all users agents know. Child elements of BODY will inherit the value of the 'font-family' property.

The second declaration sets the font size of the BODY element to 12 points. The "point" unit is commonly used in print-based typography to indicate font sizes and other length values. It's an example of an absolute unit which does not scale relative to the environment.

The third declaration uses a relative unit which scales with regard to its surroundings. The "em" unit refers to the font size of the element. In this case the result is that the margins around the BODY element are three times wider than the font size.

2.2 A brief CSS2 tutorial for XML

CSS can be used with any structured document format, for example with applications of the eXtensible Markup Language [XML10]. In fact, XML depends more on style sheets than HTML, since authors can make up their own elements that user agents don't know how to display.

Here is a simple XML fragment:

<ARTICLE>
  <HEADLINE>Fredrick the Great meets Bach</HEADLINE>
  <AUTHOR>Johann Nikolaus Forkel</AUTHOR>
  <PARA>
    One evening, just as he was getting his 
    <INSTRUMENT>flute</INSTRUMENT> ready and his
    musicians were assembled, an officer brought him a list of
    the strangers who had arrived.
  </PARA>
</ARTICLE>

To display this fragment in a document-like fashion, we must first declare which elements are inline-level (i.e., do not cause line breaks) and which are block-level (i.e., cause line breaks).

INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }

The first rule declares INSTRUMENT to be inline and the second rule, with its comma-separated list of selectors, declares all the other elements to be block-level.

One proposal for linking a style sheet to an XML document is to use a processing instruction:

<?XML:stylesheet type="text/css" href="bach.css"?>
<ARTICLE>
  <HEADLINE>Fredrick the Great meets Bach</HEADLINE>
  <AUTHOR>Johann Nikolaus Forkel</AUTHOR>
  <PARA>
    One evening, just as he was getting his 
    <INSTRUMENT>flute</INSTRUMENT> ready and his
    musicians were assembled, an officer brought him a list of
    the strangers who had arrived.
  </PARA>
</ARTICLE>

A visual user agent could format the above example as:

Example rendering   [D]

Notice that the word "flute" remains within the paragraph since it is the content of the inline element INSTRUMENT.

Still, the text isn't formatted the way you would expect. For example, the headline font size should be larger than then rest of the text, and you may want to display the author's name in italic:

INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
HEADLINE { font-size: 1.3em }
AUTHOR { font-style: italic }
ARTICLE, HEADLINE, AUTHOR, PARA { margin: 0.5em }

A visual user agent could format the above example as:

Example rendering   [D]

Adding more rules to the style sheet will allow you to further improve the presentation of the document.

2.3 The CSS2 processing model

This section presents one possible model of how user agents that support CSS work. This is only a conceptual model; real implementations may vary.

In this model, a user agent processes a source by going through the following steps:

  1. Parse the source document and create a document tree.
  2. Identify the target media type.
  3. Retrieve all style sheets associated with the document that are specified for the target media type.
  4. Annotate every element of the document tree by assigning a single value to every property that is applicable to the target media type. Properties are assigned values according to the mechanisms described in the section on cascading and inheritance.

    Part of the calculation of values depends on the formatting algorithm appropriate for the target media type. For example, if the target medium is the screen, user agents apply the visual formatting model. If the destination medium is the printed page, user agents apply the page model. If the destination medium is an aural rendering device (e.g., speech synthesizer), user agents apply the aural rendering model.

  5. From the annotated document tree, generate a formatting structure. Often, the formatting structure closely resembles the document tree, but it may also differ significantly, notably when authors make use of pseudo-elements and generated content. First, the formatting structure need not be "tree-shaped" at all -- the nature of the structure depends on the implementation. Second, the formatting structure may contain more or less information than the document tree. For instance, if an element in the document tree has a value of 'none' for the 'display' property, that element will generate nothing in the formatting structure. A list element, on the other hand, may generate more information in the formatting structure: the list element's content and list style information (e.g., a bullet image).

    Note that the CSS user agent does not alter the document tree during this phase. In particular, content generated due to style sheets is not fed back to the document language processor (e.g., for reparsing).

  6. Transfer the formatting structure to the target medium (e.g., print the results, display them on the screen, render them as speech, etc.).

Step 1 lies outside the scope of this specification (see, for example, [DOM]).

Steps 2-5 are addressed by the bulk of this specification.

Step 6 lies outside the scope of this specification.

2.3.1 The canvas

For all media, the term canvas describes "the space where the formatting structure is rendered." The canvas is infinite for each dimension of the space, but rendering generally occurs within a finite region of the canvas, established by the user agent according to the target medium. For instance, user agents rendering to a screen generally impose a minimum width and choose an initial width based on the dimensions of the viewport. User agents rendering to a page generally impose width and height constraints. Aural user agents may impose limits in audio space, but not in time.

2.3.2 CSS2 addressing model

CSS2 selectors and properties allow style sheets to refer to the following parts of a document or user agent:

2.4 CSS design principles

CSS2, as CSS1 before it, is based on a set of design principles: