If you're open to alternative dictionaries, look for one with an open
API. I know Gnome has a widget that allows you to place a dictionary in the
toolbar. You might want to find out which API they're using and use it as
well. You might be able to avoid dealing with HTML altogether if you use an
API (you'd be dealing with XML instead), and the service provider is less
likely to change the HTML formatting if they've published the API openly.

   Another thing you might try is using the Google websearch API. In
"normal" Google, if you prefix a search query with "define:", you'll get the
definition of the word, instead of pages which contain the word as keywords.
E.g. "define:dogma" gives you definitions form the word "dogma". Maybe this
facility is also accessible via Google's search API. The API devloper kit
contains sample programs in Java.

http://www.google.com/apis/

   - Oliver