I had a markup problem like the following

<ul>
 <li><span title=foo
<p>test</p>

(the list was in a separate file and that file was unexpectedly truncated inside an unquoted attribute value)

The validator messages were not so helpful in identifying the problem.

Compare with

<ul><li><span title=foo<p>test</p>

Now the validator tells me that there was an unexpected "<".

It would be useful if it did the same for "<" in attribute names.