Dig around the internet and I get to this useful and ancient article on A List Apart about custom DTDs.
Upshot – add this to the beginning of your page when validating – a quick check on Firefox suggests you can’t really have it on your live site. Ho hum.
<!DOCTYPE html PUBLIC
"-//W3C//DTD XHTML 1.0 Transitional//EN"
<!ATTLIST a track CDATA #IMPLIED>
<!ATTLIST input track CDATA #IMPLIED>