HTML Checking for Large Sites

The attribute xmlns:serif is not valid. Check this guide for more information on this issue.

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" />

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.

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>