HTML Guide
Quote characters used for attributes can use either single quotes (') or double quotes ("), and they must be properly matched, for example:
<p class="news">...</p>
A common cause for this issue is forgetting to use the equal sign (=), so the HTML parser wrongly believes the quote forms part of the attribute name, for example:
<p class "news">this is wrong</p>Related W3C validator issues
Check the syntax of the affected tag, it’s probably malformed and a < character inside has been interpreted as an attribute.
For example, this code might cause this issue:
<!-- Malformed img tag -->
<img src="photo.jpg" alt="smiling cat" < />
<!-- Fixed img tag -->
<img src="photo.jpg" alt="smiling cat" />The attribute xmlns:serif is not valid. Check this guide for more information on this issue.
An attribute could not be parsed from the HTML input, probably due to a typo. Check this guide for a related HTML issue.
An invalid attribute has been found on an element. Check the affected tag to ensure attributes are well-formed, and if they are you can consider using custom data attributes.