As of today, there is little or no certification for Web professionals, and only few universities teach Web technologies, leaving most Web-smiths to learn by themselves, with varied success.

Seasoned, able professionals will take pride in creating Web content using semantic and well-formed markup, separation of style and content, etc.

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Validation is one of the simplest ways to check whether a page is built in accordance with Web standards, and provides one of the most reliable guarantee that future Web platforms will handle it as designed.

It is reasonable to consider that standards such as HTML and CSS are a form of “coding style” which is globally agreed upon.

Beginners and students, on the other hands, will find automated checking tools invaluable in spotting mistakes.

Some teachers also stress that automated validation tests are a good introduction to broader, more complex quality concepts such as accessibility.

This is deliberate, and doesn't imply any kind of browser bug.

A term sometimes used for this is WYSINWOG - What You See Is Not What Others Get (unless by coincidence).

Validation can then be used as a quick check to determine whether the code is the clean work of a seasoned HTML author, or quickly hacked-together tag soup.

Validation, as any process of debugging code, is sometimes difficult, and the vast improvements in automatic error correction has made modern browser cope very well with errors in HTML or CSS.

Using standard, interoperable markup and stylesheets, on the other hand, offers a much greater chance of having one's page handled consistently across platforms and user-agents.