From RDFaWiki

Using an XHTML Validator on your Web Pages

RDFa extractors will usually try their best to extract correct information from a corrupt XHTML document. Sometimes your XHTML will be so corrupt that the RDFa extractor may generate incorrect triples, therefore it is best to use an XHTML validator. Read more on how to validate.

Explanation

XHTML documents contain tags, and when all of the tags match up in the document, the state of the document is known as "well formed". Here is an example of a well-formed piece of XHTML:

<p>This is a paragraph</p>
<p>This is the second paragraph</p>

However, when tags get mis-matched or mis-aligned, the document is not well formed. Here is an example of a non-well-formed piece of XHTML:

<p>This is a paragraph
<p>This is the second paragraph</p>

Note that the first paragraph end tag is missing. It is never a good idea to have non-well-formed XHTML because you don't know how software programs reading that XHTML will react. Most will attempt to correct the mistake, but there are documents that are so badly formed that the recovery process loses a great deal of data in the document. This means not only loss of visual content, but loss of RDFa triples as well.

Other Validators

• List other validators here...