This test page shows how to extend the XHTML DTDs to include custom attributes. This page also shows the main browser problem.
This paragraph has an attribute behavior="mouseover". Normally the W3C
validator would not declare this page valid, but because of the custom DTD it now accepts this extra attribute.
Try it.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" [ <!ATTLIST p behavior CDATA #IMPLIED> ]>
The extra <!ATTLIST p behavior CDATA #IMPLIED> tells the validator that in this document
the <p> elements may have an attribute behavior.
Unfortunately, though the validator accepts the page, the browsers don't. With the exception of Opera they
think that the > of the !ATTLIST closes the entire <!DOCTYPE> tag. This
is not illogical: of old browsers weren't equipped to handle nested tags.
The result is unpleasant though: all browsers but Opera start this page with:
]>
To me this is a bug: browsers should be able to read such customized doctypes.