When you send XHTML to a browser using the common
text/html content type, all major browsers will respond by using their regular HTML parsers on your page, regardless of the doctype. For some reason, the W3C HTML Validator doesn’t follow this widely-accepted convention. Instead, if you’re using an XHTML doctype, the W3C Validator will use an XML parser on your page. Obviously, that will give different results than what your browser is seeing. And unfortunately, there was no easy way to force the W3C Validator to parse your page with an HTML parser like everyone else did.
That is, until now. I’ve just released the new Validate XHTML Parsed as HTML tool. It works very much like the HTML Good Practice Checker: you submit the URL you want to test, it makes a few minimal changes to the beginning of your markup in order to modify how the W3C Validator sees your code, you click the button to validate it, and the results appear below.
The purpose of this tool is to illustrate how the compatibility issues between XHTML and HTML are not as simple as whether or not you follow the HTML Compatibility Guidelines. A fully-compliant HTML parser following widely-accepted conventions for parsing mode selection would encounter all of these errors when attempting to parse your page. Popular web browsers don’t support the Null End Tag construct, so they would see it slightly differently, but they would still see errors in each instance of
/> on the page. I thought one of the selling qualities of XHTML was that it was supposed to put an end to lax error handling. I guess not.