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, h
. 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. element is obsoleted, because making it conforming would make anti- standardistas think that the HTML5 people have gone crazy, which could lead to bad PR. is obsoleted mainly as a matter of principle of not giving special markup to one particular plug-in. The classid attribute on is obsoleted, because it’s in practice ActiveX-specific.name attribute on and the language attribute on .(I develop the Validator.nu HTML5 validator which is also the HTML5 validation engine used by the W3C validator.)