In the early days of XHTML many sites had problems where code was declared as HTML or XHTML when in fact there was a mixture of the two in both cases.  If Sitemorse had reported every infringement then most sites would have been reported as having thousands of errors under the Code Quality category and people would have felt overwhelmed by the volume of errors we were reporting.  As most browsers coped well with these errors and rendered the pages OK we didn't want to be in a position where either people gave up on Code Quality or were spending ages clearing problems that had no real impact on their sites.  With the volume of errors that would have been reported there was a definite possibility of being in a situation where people "couldn't see the wood for the trees."

We've kept an eye on this and now feel we can be more picky about the differences between HTML and XHTML.  Having previously been fairly relaxed about the use of XHTML features in HTML, and vice versa, we're now making a proper distinction between the two as most sites seem to have got the distinction correct nowadays.

An example of what might typically come up in reports is to do with "empty" tags, e.g. "<br>". These tags do not contain any content, and in HTML they should not have close-tags. In XHTML, all tags should have close-tags, so they must be written "<br />".

So in short:
HTML: <br>
XHTML: <br />

Other tags that this issue often crops up with include, for example, <link>, <meta>, <img>, <input> and <hr>.

The bottom line is if you declare that the code is HTML make sure it is, likewise for XHTML.