Attribute “<” not allowed on element “X” at this point.

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" />