The Ultimate CSS Reference

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Tommy Olsson and Paul O'Brien

"The Ultimate CSS Reference"

Figure 1.1: The three layers of a web document
The behavior layer involves real-time user interaction with the document. This
task is normally handled by JavaScript. The interaction can be anything from a
trivial validation that ensures a required field is filled in before an order form can
be submitted, to sophisticated web applications that work much like ordinary
desktop programs.
It??™s possible to embed all three layers within the same document, but keeping them
separate gives us one valuable advantage: we can modify or replace any of the layers
without having to change the others.
Certain versions of HTML and XHTML also contain presentational element
types??”that is, elements that specify the appearance of the content, rather than
structure or semantics. For example, and can be used to control the
presentation of text, and
will insert a visible rule element. However, as these
types of elements embed presentation-layer information within the content layer,
they negate any advantage we may have gained by keeping the layers separate.

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Tommy Olsson and Paul O'Brien

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