Interesting.. when Blogger auto-inserts <br> elements for linebreaks, they use the XHTML syntax <br />. They don't know whether your page is HTML or XHTML, so they just assume XHTML.

The resulting HTML validates and also renders as you would expect in Mozilla, Opera, and MSIE. But it still doesn't seem right to mix up HTML and XHTML like that.

See also Sending XHTML as text/html Considered Harmful.