Best Way to View Generated Source of Webpage?

[updating in response to more details in the edited question]

The problem you're running into is that, once a page is modified by ajax requests, the current HTML exists only inside the browser's DOM-- there's no longer any independent source HTML that you can validate other than what you can pull out of the DOM.

As you've observed, IE's DOM stores tags in upper case, fixes up unclosed tags, and makes lots of other alterations to the HTML it got originally. This is because browsers are generally very good at taking HTML with problems (e.g. unclosed tags) and fixing up those problems to display something useful to the user. Once the HTML has been canonicalized by IE, the original source HTML is essentially lost from the DOM's perspective, as far as I know.

Firefox most likley makes fewer of these changes, so Firebug is probably your better bet.

A final (and more labor-intensive) option may work for pages with simple ajax alterations, e.g. fetching some HTML from the server and importing this into the page inside a particular element. In that case, you can use fiddler or similar tool to manually stitch together the original HTML with the Ajax HTML. This is probably more trouble than it's worth, and is error prone, but it's one more possibility.

[Original response here to the original question]

Fiddler (http://www.fiddlertool.com/) is a free, browser-independent tool which works very well to fetch the exact HTML received by a browser. It shows you exact bytes on the wire as well as decoded/unzipped/etc content which you can feed into any HTML analysis tool. It also shows headers, timings, HTTP status, and lots of other good stuff.

You can also use fiddler to copy and rebuild requests if you want to test how a server responds to slightly different headers.

Fiddler works as a proxy server, sitting in between your browser and the website, and logs traffic going both ways.

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