Essentially, not all tags "require" a closing tag when using the HTML
dtd (not XHTML) - does HAML have a mode where he ignores these closing
tags, and thus makes the file size smaller?

Haml does indeed omit the closing tags of self-closing tags such as "br" and
"meta" in HTML mode.

No - I don't think Haml will ever do any optimizations that will actually
change the rendering of a document. Consider for example

#foo
  %p bar
  baz

If we removed </p>, this would render as

<div id="foo">
  <p>bar
  baz
</div>

which is equivalent to

<div id="foo">
  <p>bar
  baz</p>
</div>

and *not* equivalent to

<div id="foo">
  <p>bar</p>
  baz
</div>

Unless you can come up with a way of removing the tags that is guaranteed
never to produce this sort of change, it's not safe to include it.

Here's a proof of concept (monkeypatch):http://gist.github.com/137005

*Keep in mind this is still vulnerable to the issue Nathan explains:
*