Justin is dead on. The key point here is that HTML is just a language for describing a document. Once the browser reads it, it's gone. Open tags, close tags, and formatting are all taken care of by the parser and then go away. Any tool that shows you HTML is generating it based on the contents of the document, so it will always be valid.
I had to explain this to another web developer once, and it took a little while for him to accept it.
You can try it for yourself in any JavaScript console:
el = document.createElement('div');
el.innerHTML = "<p>Some text<P>More text";
el.innerHTML; // <p>Some text</p><p>More text</p>
The un-closed tags and uppercase tag names are gone, because that HTML was parsed and discarded after the second line.
The right way to modify the document from JavaScript is with document methods (createElement, appendChild, setAttribute, etc.) and you'll observe that there's no reference to tags or HTML syntax in any of those functions. If you're using document.write, innerHTML, or other HTML-speaking calls to modify your pages, the only way to validate it is to catch what you're putting into them and validate that HTML separately.
That said, the simplest way to get at the HTML representation of the document is this:
document.documentElement.innerHTML