HTML Guide

“X” in an unquoted attribute value. Probable causes: Attributes running together or a URL query string in an unquoted attribute value.

Attribute values for HTML elements must be enclosed within quotes. Check for attributes whose values are missing quotes.

Example:

<!-- These are invalid -->
<div class=important></div>
<div class=important"></div>

<!-- This is valid -->
<div class="important"></div>