2 Introduction to CSS2
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:
- the type of link: to a "stylesheet".
- the location of the style sheet via the "ref" attribute.
- the type of style sheet being linked: "text/css".
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.
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:
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:
Adding more rules to the style sheet will allow you to further
improve the presentation of the document.
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:
- Parse the source document and create a document tree.
- Identify the target media type.
- Retrieve all style sheets associated with the document that are
specified for the target media type.
- 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.
- 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).
- 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.
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.
CSS2 selectors and properties allow
style sheets to refer to the following parts of a document
or user agent:
- Elements in the document tree and certain relationships between
them (see the section on selectors).
- Attributes of elements in the document tree, and values of those
attributes (see the section on attribute selectors).
- Some parts of element content (see the :first-line and :first-letter pseudo-elements.
- Elements of the document tree when they are in a certain state
(see the section on pseudo-classes).
- Some aspects of the canvas where
the document will be rendered.
- Some system information (see the section on user
interface).
CSS2, as CSS1 before it, is based on a set of design principles:
-
Forward and backward compatibility. CSS2 user
agents will be able to understand CSS1 style sheets. CSS1 user agents
will be able to read CSS2 style sheets and discard parts they don't
understand. Also, user agents with no CSS support will be able to
display style-enhanced documents. Of course, the stylistic
enhancements made possible by CSS will not be rendered, but all
content will be presented.
-
Complementary to structured documents. Style
sheets complement structured documents (e.g., HTML and XML
applications), providing
stylistic information for the marked-up text. It should be easy to
change the style sheet with little or no impact on the markup.
-
Vendor, platform, and device independence. Style
sheets enable documents to remain vendor, platform, and device
independent. Style sheets themselves are also vendor and platform
independent, but CSS2 allows you to target a style sheet for a group of
devices (e.g., printers).
-
Maintainability. By pointing to style sheets from
documents, webmasters can simplify site maintenance and retain
consistent look and feel throughout the site. For example, if
the organization's background color changes, only one file needs to be
changed.
-
Simplicity. CSS2 is more complex than CSS1, but
it remains a simple style language which is human readable and
writable. The CSS properties are kept independent of each other to the
largest extent possible and there is generally only one way to achieve a
certain effect.
-
Network performance. CSS provides for compact
encodings of how to present content. Compared to images or audio
files, which are often used by authors to achieve certain rendering
effects, style sheets most often decrease the content size. Also,
fewer network connections have to be opened which further increases
network performance.
-
Flexibility. CSS can be applied to content in
several ways. The key feature is the ability to cascade style
information specified in the default (user agent) style sheet, user
style sheets, linked style sheets, the document head, and in
attributes for the elements forming the document body.
-
Richness. Providing authors with a rich set of
rendering effects increases the richness of the Web as a medium of
expression. Designers have been longing for functionality commonly
found in desktop publishing and slide-show applications. Some of
the requested rendering effects conflict with device independence, but
CSS2 goes a long way toward granting designers their requests.
-
Alternative language bindings. The set of CSS
properties described in this specification form a consistent
formatting model for visual and aural presentations. This formatting
model can be accessed through the CSS language, but bindings to other
languages are also possible. For example, a JavaScript program may
dynamically change the value of a certain element's 'color' property.
-
Accessibility. Several CSS
features will make the Web more accessible
to users with disabilities:
- Properties to control font appearance allow authors
to eliminate inaccessible bit-mapped text images.
- Positioning properties allow authors to eliminate
mark-up tricks (e.g., invisible images) to force layout.
- The semantics of
!important rules mean that users with
particular presentation requirements
can override the author's style sheets.
- The new 'inherit' value for all properties
improves cascading generality and allows for
easier and more consistent style tuning.
- Improved media support, including media groups and the
braille, embossed, and tty media types, will allow users and
authors to tailor pages to those devices.
- Aural properties give control over voice and audio output.
- The attribute
selectors, 'attr()' function,
and 'content' property give access to alternate content.
- Counters and section/paragraph numbering can improve
document navigability and save on indenting spacing
(important for braille devices). The 'word-spacing' and
'text-indent' properties also eliminate the need
for extra whitespace in the document.
Note. For more information
about designing accessible documents using CSS and HTML, please
consult [WAI-PAGEAUTH].
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About the CSS2 Specification
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