&lt;hr/> is a valid xhtml element. Fossil says it is not.
However &lt;hr /> and &lt;hr id='hrid'/> are considered valid.

The problem is in wikiformat.c:markupLength in the line

  while( isalnum(z[n]) ){ n++; }

==>  if( (c = z[n])!='>' && !isspace(c) ) return 0;

  while( (c = z[n])!=0 && (c!='>' || inparen) ){