Not everyone who accesses the Web can see the effects we've discussed in this book. Some 1.1 million people in the United States are blind, and they have a very different experience of the Web than sighted persons. Fortunately, CSS is not silent on the matter of non-visual access. CSS2 included the ability to apply styles in nonlo-screen media. While most of the Web's evolution has taken place on monitorsthat is to say, in a visual mediumCSS2 can be used in non-visual media, assuming that the user agent has proper support.   Read more

Pseudo-Classes and Pseudo-Elements.These kind of selectors allow that styles are applied to pieces of a document, based on something else than the structure of the document. The styles are based on circumstances that can not be predicted in advance. Pseudo-classes and pseudo-elements can be used in CSS selectors, but do not exist in the HTML source.  Read more

Selectors are one of the most important aspects of CSS as they are used to "select" elements on an HTML page so that they can be styled. Selectors are only case-sensitive if the document language is also case-sensitive (eg: HTML: not case-sensitive, XML: case-sensitive), ID selectors have a higher cascade specificity than an equivalent attribute selector (eg: the selector '#foo' has a higher specificity than '*[id="foo"]'   Read more

There may be times when you want your document to mimic the user's computing environment as closely as possible. An obvious example is if you're creating web-based applications, where the goal is to make the web component seem like a part of the user's operating system. While CSS2 doesn't make it possible to reuse every last aspect of the operating system's appearance in your documents, you can choose from a wide variety of colors and a short list of fonts.  Read more

Length Units Many CSS properties, such as margins, depend on length measurements to properly display various page elements. It's no surprise, then, that there are a number of ways to measure length in CSS. All length units can be expressed as either positive or negative numbers followed by a label (although some properties will accept only positive numbers). You can also use real numbersthat is, numbers with decimal fractions, such as 10.5 or 4.561. All length units are followed by a two-letter abbreviation that represents the actual unit of length being specified, such as in (inches) or pt (points). The only exception to this rule is a length of 0 (zero), which need not be followed by a unit.   Read more

As important as specificity may be to understanding how declarations are applied to a document, another key concept is inheritance. Inheritance is the mechanism by which styles are applied not only to a specified element, but also to its descendants. If a color is applied to an H1 element, for example, then that color is applied to all text in the h1, even the text enclosed within child elements of that h1.  Read more