This is deliberate. HTML browsers accept any input, correct
or incorrect, and try to make something sensible of it. This
error-correction makes browsers very hard to write,
especially if all browsers are expected to do the same
thing. It has also meant that huge numbers of HTML documents
are incorrect, because since they display OK in the browser,
the author isn't aware of the errors. This makes it
incredibly difficult to write new web user agents since
documents claiming to be HTML are often so poor.