User Agent: Mozilla/5.0 (X11; FreeBSD amd64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/38.0.2125.111 Safari/537.36 Steps to reproduce: Currently firefox (and other browsers) is very liberal with html syntax, and tries to interpret everything it encounters. Really bad html syntax problems go unnoticed, since browser doesn't complain about them. Examples of such blatant errors I encountered: 1. unbalanced tags: <h1>some text</h2> 2. missing closing tag: <table><div>some text</table> 3. stray characters in the middle of tags: <a href="http://some-site.com";>some text</a> (stray comma) 4. nonexistent tags: <emframe src="http://rcm-na.amazon-adsystem.com"></iframe> 5. broken tags(no closing '>'): <li data-target="#headliners" data-slide-to="0" class="active"</li> I suggest Firefox adds red-colored "Error" symbol in the right side of the URL box, clicking on which will show the list of such errors in the page. This will prominently notify page authors/owners that something isn't quite right in their pages. Currently there is no indication of problems, users stay unaware, and this allows such problems to stay up. This also can he considered the standard compliance suggestion, because currently firefox silently allows significant deviations from the html standard.
Interesting suggestion. In the meantime, there are several addons that will validate html.
Component: Untriaged → Developer Tools
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