| abstract
| - The Separation of Layers within an HTML document is a somewhat recent convention within the Web design community. It is centered on standards for the ultimate purposes, shared with all other , of cross-browser compatibility and a concept of the ideal functionality for the Web. The focus is more on the latter for this specific convention, but cross-browser compatibility augments how that goal is achieved, to the point of making exceptions where necessary. The specific ideal behind it is the separation of concerns via modular programming, i.e. the maximal independence of each piece of software, or module, from each other piece. A module, in any sub-field of programming, typically follows the Unix philosophy to "do one thing well" and is therefore as small as it can be while still doing its job well. ("Programming" here is used loosely, although the use of "software" for HTML and CSS is technically accurate.) The modules in this case are HTML (content), CSS (style), and JavaScript (behavior). When a server-side scripting language such as PHP is available, content is often further separated into structure (where content is placed in a document) and semantics (the meaning of bits of content on a minute scale).
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