Producing valid HTML pages can be a difficult task. But since every web page has to obey some rules (a.k.a. standards), its structure can be validated either by the excellent W3C Markup Validation Service (free online service) or by using some browser extensions.
There is a very helpful extension for Firefox called HTML Validator, which validates each page by using W3C Tidy. A little icon at the lower right corner of Firefox shows the result for each page and it can be in one of three states: Valid, Warning, Error. Double-clicking on the icon reveals the source of the page, highlights any lines/tags with warnings/errors and provides the option to “Clean up the page…”
Another highly recommended extension for Firefox is the Web Developer extension which, among many other things, eases the validation process for HTML pages, CSS (cascading style sheets), links and feeds.
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