HTML Guide
Your HTML markup contains an end tag for X, but there are nested open elements that need to be closed first. For example, <li><span>example</li> is invalid because you need to close the <span> tag before you close the <li>. This would be valid: <li><span>example</span></li>.
Related W3C validator issues
A closing tag </li> has been found, but there were open elements nested inside the <li>. You need to close the nested elements before you close the <li>.
A closing tag </li> has been found, but there were open elements nested inside the <li>. You need to close the nested elements before you close the <li>.
For example:
<li>
<span>example
</li>The above is invalid because you need to close the <span> tag before you close the <li>.
This would be valid:
<li>
<span>example</span>
</li>An end tag </p> has been found that cannot be matched for an opening tag <p>. Most of the times this is due to closing the same tag twice, for example:
<p>some text</p></p>And end tag for element X has been found with no corresponding opening tag for it. Most of the times this is due to closing the same tag twice.