This tool will validate your XHTML document using an HTML parser and the XHTML DTD. In theory, this is how common user agents see your XHTML document when it's sent with the
text/html content type. Internet Explorer, Firefox, Safari, and Opera all use an HTML parser under this condition, although the W3C HTML Validator typically uses an XML parser, which gives misleading results. This tool removes XML declarations and makes a slight modification to the doctype in order to trigger the HTML parser in the W3C Validator. The actual contents of the DTD are identical to the official XHTML DTDs from the W3C.
Even if a document is valid XHTML and valid HTML, the different parser types may generate different parse trees, so it still may not be compatible both ways. The most common difference is that when a fully-compliant HTML parser comes across a tag like
<br />, it will treat the
> as a displayable character on the page. Other differences, such as CSS and scripting, are not tested here.