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18.1 Styles and Style Sheets with CSSStyles in CSS are specified as a semicolon-separated list of name/value attribute pairs, where each name and value are separated by colons. For example, the following style specifies bold, blue, underlined text: font-weight: bold; color: blue; text-decoration: underline; The CSS standard defines quite a few style attributes you can set. Table 18-1 lists all the attributes except for those used only in audio style sheets. You are not expected to understand or be familiar with all these attributes, their values, or their meanings. As you become familiar with CSS and use it in your documents and scripts, however, you may find this table a convenient quick reference. For more complete documentation on CSS, consult Cascading Style Sheets: The Definitive Guide, by Eric Meyer (O'Reilly), or Dynamic HTML: The Definitive Guide, by Danny Goodman (O'Reilly). Or read the specification itself -- you can find it at http://www.w3c.org/TR/REC-CSS2/. The second column of Table 18-1 shows the allowed values for each style attribute. It uses the grammar used by the CSS specification. Items in fixed-width font are keywords and should appear exactly as shown. Items in italics specify a data type such as a string or a length. Note that the length type is a number followed by a units specification such as px (for pixels). See a CSS reference for details on the other types. Items that appear in italic fixed-width font represent the set of values allowed by some other CSS attribute. In addition to the values shown in the table, each style attribute may have the value "inherit", to specify that it should inherit the value from its parent. Values separated by a | are alternatives; you must specify exactly one. Values separated by || are options; you must specify at least one, but you may specify more than one, and they can appear in any order. Square brackets [] are used for grouping values. An asterisk (*) specifies that the previous value or group may appear zero or more times, a plus sign (+) specifies that the previous value or group may appear one or more times, and a question mark (?) specifies that the previous item is optional and may appear zero or one time. Numbers within curly braces specify a number of repetitions. For example, {2} specifies that the previous item must be repeated twice, and {1,4} specifies that the previous item must appear at least once and no more than four times. (This repetition syntax may seem familiar: it is the same one used by JavaScript regular expressions, discussed in Chapter 10.)
The CSS standard allows certain style attributes that are commonly used together to be combined using special shortcut attributes. For example, the font-family, font-size, font-style, and font-weight attributes can all be set at once using a single font attribute: font: bold italic 24pt helvetica; In fact, some of the attributes listed in Table 18-1 are themselves shortcuts. The margin and padding attributes are shortcuts for attributes that specify margins, padding, and borders for each of the individual sides of an element. Thus, instead of using the margin attribute, you can use margin-left, margin-right, margin-top, and margin-bottom, and similarly for padding. 18.1.1 Applying Style Rules to Document ElementsYou can apply style attributes to the elements of a document in a number of ways. One way is to use them in the style attribute of an HTML tag. For example, to set the margins of an individual paragraph, you can use a tag like this: <p style="margin-left: 1in; margin-right: 1in;"> One of the important goals of CSS is to separate document content and structure from document presentation. Specifying styles with the style attribute of individual HTML tags does not accomplish this (although it can be a useful technique for DHTML). To achieve the separation of structure from presentation, we use style sheets, which group all the style information into a single place. A CSS style sheet consists of a set of style rules. Each rule begins with a selector that specifies the document element or elements to which it applies, followed by a set of style attributes and their values within curly braces. The simplest kind of rule defines styles for one or more specific tag names. For example, the following rule sets the margins and background color for the <body> tag: body { margin-left: 30px; margin-right: 15px; background-color: #ffffff }
The following rule specifies that text within <h1> and <h2> headings should be centered: h1, h2 { text-align: center; }
In the previous example, note the use of a comma to separate the tag names to which the styles are to apply. If the comma is omitted, the selector specifies a contextual rule that applies only when one tag is nested within another. For example, the following rules specify that <blockquote> tags are displayed in an italic font, but text inside an <i> tag inside a <blockquote> is displayed in plain, nonitalic text: blockquote { font-style: italic; }
blockquote i { font-style: normal; }
Another kind of style sheet rule uses a different selector to specify a class of elements to which its styles should be applied. The class of an element is defined by the class attribute of the HTML tag. For example, the following rule specifies that any tag with the attribute class="attention" should be displayed in bold: .attention { font-weight: bold; }
Class selectors can be combined with tag name selectors. The following rule specifies that when a <p> tag has the class="attention" attribute, it should be displayed in red, in addition to being displayed in a bold font (as specified by the previous rule): p.attention { color: red; }
Finally, style sheets can contain rules that apply only to individual elements that have a specified id attribute. The following rule specifies that the element with an id attribute equal to "p1" should not be shown: #p1 { visibility: hidden; }
We've seen the id attribute before: it is used with the DOM function getElementById( ) to return individual elements of a document. As you might imagine, this kind of element-specific style sheet rule is useful when we want to manipulate the style of an individual element. Given the previous rule, for example, a script might switch the value of the visibility attribute from hidden to visible, causing the element to dynamically appear. 18.1.2 Associating Style Sheets with DocumentsYou can incorporate a style sheet into an HTML document by placing it between <style> and </style> tags within the <head> of the document, or you can store the style sheet in a file of its own and reference it from the HTML document using a <link> tag. You can also combine these two techniques by creating a document-specific style sheet between <style> tags that references or imports a document-independent style sheet using the special @import "at-rule." Consult a CSS reference for details on @import. 18.1.3 The CascadeRecall that the C in CSS stands for "cascading." This term indicates that the style rules that apply to any given element in a document can come from a cascade of different sources. Each web browser typically has its own default styles for all HTML elements and may allow the user to override these defaults with a user style sheet. The author of a document can define style sheets within <style> tags or in external files that are linked in or imported into other style sheets. The author may also define inline styles for individual elements with the HTML style attribute. The CSS specification includes a complete set of rules for determining which rules from the cascade take precedence over the other rules. Briefly, however, what you need to know is that the user style sheet overrides the default browser style sheet, author style sheets override the user style sheet, and inline styles override everything. The exception to this general rule is that user style attributes whose values include the !important modifier override author styles. Within a style sheet, if more than one rule applies to an element, styles defined by the most specific rule override conflicting styles defined by less specific rules. Rules that specify an element id are the most specific. Rules that specify a class are next. Rules that specify only tag names are the least specific, but rules that specify multiple nested tag names are more specific than rules that specify only a single tag name. 18.1.4 Versions of CSSAt the time of this writing, there are two versions of the CSS standard. CSS1 was adopted in December, 1996 and defines attributes for specifying colors, fonts, margins, borders, and other basic styles. Netscape 4 and Internet Explorer 4 both implement at least partial support for CSS1. The second edition of the standard, CSS2, was adopted in May, 1998; it defines a number of more advanced features, most notably support for absolute positioning of elements. The advanced features of CSS2 are supported only in sixth-generation browsers. Fortunately, however, the crucial positioning features of CSS2 began the standardization process as part of a separate CSS-Positioning (CSS-P) effort, and therefore some of these DHTML-enabling features are available in fourth-generation browsers. Work continues on a third edition of the CSS standard. You can find the CSS specifications and working drafts at http://www.w3.org/Style/CSS/. 18.1.5 CSS ExampleExample 18-1 is an HTML file that defines and uses a style sheet. It demonstrates the previously described tag name, class, and ID-based style rules, and it also has an example of an inline style defined with the style attribute. Remember that this example is meant only as an overview of CSS syntax and capabilities. Full coverage of CSS is beyond the scope of this book. Example 18-1. Defining and using Cascading Style Sheets<head>
<style type="text/css">
/* Specify that headings display in blue italic text. */
h1, h2 { color: blue; font-style: italic }
/*
* Any element of class="WARNING" displays in big bold text with large margins
* and a yellow background with a fat red border.
*/
.WARNING {
font-weight: bold;
font-size: 150%;
margin: 0 1in 0 1in; /* top right bottom left */
background-color: yellow;
border: solid red 8px;
padding: 10px; /* 10 pixels on all 4 sides */
}
/*
* Text within an h1 or h2 heading within an element with class="WARNING"
* should be centered, in addition to appearing in blue italics.
*/
.WARNING h1, .WARNING h2 { text-align: center }
/* The single element with id="P23" displays in centered uppercase. */
#P23 {
text-align: center;
text-transform: uppercase;
}
</style>
</head>
<body>
<h1>Cascading Style Sheets Demo</h1>
<div class="WARNING">
<h2>Warning</h2>
This is a warning!
Notice how it grabs your attention with its bold text and bright colors.
Also notice that the heading is centered and in blue italics.
</div>
<p id="P23">
This paragraph is centered<br>
and appears in uppercase letters.<br>
<span style="text-transform: none">
Here we explicitly use an inline style to override the uppercase letters.
</span>
</p>
</body>
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