HTML Guide for strong

An end tag </strong> has been found in an incorrect place within the document, violating nesting rules. A common case is closing it before closing other nested tags, for example:

<!-- This line is incorrect as the <strong> tag was closed before the nested <a> tag -->
<strong><a href="#">link</strong></a>

<!-- This line is OK as every end tag respects the nesting rules -->
<strong><a href="#">link</a></strong>