createElement('a:b') results in an element with the local name 'b'. It should be 'a:b'. (DOM3Core says that localName should be null, but that's nonsense. In Firefox and Opera localName is 'a:b'.) Test cases at URL.
This works for me in ToT. Testing with nightlies shows that this was fixed between r31224 and r31232 for Acid3, but this specific case doesn't seem to be covered by any regression tests.
Landed test cases in <http://trac.webkit.org/changeset/34301>.
Mass moving XML DOM bugs to the "DOM" Component.