 <p>The HTML5 spec, <a href="http://www.w3.org/TR/html5/syntax.html#attribute-name-state" rel="nofollow"><strong>Section 8.2.4.35 - 'Attribute name state'</strong></a> says:</p> <blockquote> <p>When the user agent leaves the attribute name state (and before emitting the tag token, if appropriate), the complete attribute's name must be compared to the other attributes on the same token; if there is already an attribute on the token with the exact same name, then this is a parse error and the new attribute must be removed from the token.</p> </blockquote> <p>So, to answer your question, it's invalid HTML.</p>