how to get intellij editor to resolve xhtml elements in an xsl document

Trying to edit an xsl document in intellij and i get
"cannot resolve symbol " for all my html elements.

Can anyone tell me how to tell intellij to resolve these ?

I tried mapping http://www.w3.org/1999/xhtml with
the file xhtml1-strict.dtd that I downloaded from the
w3, but it does not work.

All help appreciated.