You can guess sometimes, but there is no way of knowing where the right place for it is.

in the example,<p> foo <p> bar, you can see where the </p>'s should go, because you can't nest p tags but if you have <span style="rly">Oh, rly<span style="ya">ya, rly there is no real way of knowing where the </span>'s should go, because they can legally be nested.

You'll most likely have to write rules for how (and where) to end each tag, so that you don't mess the nesting of things (like finding your whole document in a <a href="foo"> or something)