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It's widely considered that the best reason to validate one's HTML is to ensure that all browsers will treat it consistently and predictably.
The HTML 5 draft, however, contains two specifications in one. First an author spec, describing the elements and attributes that HTML authors should use, and their interrelationships. Validation of an HTML 5 page is based on this spec. The elements and attributes included are not directly drawn from HTML 4, but have needed to be justified from first principles, which means that some HTML 4 features, such as the summary attribute on <table>, longdesc on <img> and the profile attribute on <head>, do not currently appear in this draft. Such features are not considered deprecated, they are simply not included. (Their absence from the draft remains a matter of dispute, although their inclusion any time soon does not seem likely.)
Second, the draft defines a browser processing specification that seeks to define exactly how a browser's parser will treat any byte stream it's given, regardless of how well formed and valid the HTML. This means that when the browsers fully support HTML 5, it will be possible to predict how any browser will treat HTML for a much wider range of inputs than merely those that pass validation.
In particular, because HTML 5 is defined to be 100% backward compatible with today's web, all valid HTML 4, and all invalid but commonly used mark-up, will continue to be processed exactly the same as it is today, regardless of whether it is HTML 5 valid or not.
Therefore, at the very minimum, anyone using any feature from HTML 5, HTML 4, or any previous version of HTML, plus many proprietary extensions, can be confident that their HTML will get consistent and predictable treatment across all browsers.
Given this, does it make any sense to limit ones HTML 5 to that which will validate, and what practical benefit will we get from doing so?
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Google Says Valid HTML Doesn't Matter “Although we do recommend using valid HTML, it's not likely to be a factor in how Google crawls and indexes your site.”
Validation improves usability and functionality because your users are less likely to run into errors when displayed on browsers compared to non-validated websites. Validation is fully compatible with a wide range of dynamic pages, scripting, active content, and multimedia presentations.
It appears that validation is a non-factor in SEO, as long as the page renders correctly in the browser.
<!DOCTYPE html>. The reason why one would want to comply here is getting the standards mode in the easiest way possible. It’s something you can memorize unlike the HTML 4.01 or XHTML 1.0 doctypes you need to look up and copy and paste each time. Of course, the reason why you’d want the standards mode is fewer surprises on the CSS layer.longdesc, summary and profile. (Note that people disagree on whether these are, indeed, waste of time, but as currently drafted, HTML5 makes them obsolete.) That is, if you have limited resources to improve accessibility, your limited resources are better spent on something other than longdesc and summary. If you have limited resources for semantic purity, your resources are better spent on something other than making sure you have the right incantation in profile.<font> element is obsoleted, because making it conforming would make anti-<font> standardistas think that the HTML5 people have gone crazy, which could lead to bad PR. <applet> is obsoleted mainly as a matter of principle of not giving special markup to one particular plug-in. The classid attribute on <object> is obsoleted, because it’s in practice ActiveX-specific.name attribute on <a> and the language attribute on <script>.(I develop the Validator.nu HTML5 validator which is also the HTML5 validation engine used by the W3C validator.)
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