This site use HTML, not XHTML. </br> is not a valid HTML tag.

To quote from Forcing a line break: the BR element:

<!ELEMENT BR - O EMPTY                 -- forced line break -->
<!ATTLIST BR
  %coreattrs;                          -- id, class, style, title --
  >
[download]

Start tag: required, End tag: forbidden

Note the "End tag: forbidden".

Even if the </br> tag was allowed, why would you want to include it? You're not allowed to put anything in between the the <br> tag and the </br> tag. It is explicitely forbidden by the "EMPTY" in the quoted text.

You seem to be confusing BR for DIV.

<p>foo<br>bar<br>bla</p>     # ok
<p>foo<br>bar<br>bla         # ok, same as previous
<p>foo<div>bar</div>bla</p>  # ok
<p>foo<div>bar</div>bla      # ok, same as previous
[download]

Or did you mean <br/>? <br/> is an XHTML shortcut for <br></br>. Neither the shortcut nor the expanded version is valid HTML.

Sending XHTML when claiming to send HTML is bad. They are not compatible.

Summary:

• HTML: <br> (case-insensitive)
• XHTML: <br></br> (case-sensitive, nothing in between)
• XHTML shortcut: <br/> (case-sensitive)
• Perlmonks uses HTML.
• Sending XHTML as HTML is bad.

Update: Added Summary.

Update: I forgot to mention that < XXX> and < /XXX> (for whatever XXX) are invalid HTML and XHTML tags. No space is allowed after the opening <. IE and Firefox interpret the < literally.