On around 99% of pages using an xhtml dtd such as the one above, the page itself is sent as 'text/html' meaning you're not utilizing the benefits of HTML. The DTD only serves to remind the browser to render the page in standards mode.
Some text editors can make a blank html page with an auto DTD and such - for VIM I have a special function which makes an html page with a dtd, blank script element and body.
I would advise you to not use that dtd and go instead with an Html 5 dtd:
Or HTML 4 strict:
The trick though, is that the only difference between them is the part after "//W3C//DTD" and the URL to the .dtd file ( which isnt really parsed at all ).
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"