JavaScript can be disabled by users so have to allow for that
Advanced Features of DHTML often not available in some browsers so have to allow for that also.
Neither should really be called Programming Language by the way
JavaScript is a Client Side Scripting language
DHtml is only a Document Markup language. and Wasistdas ,DHTML is NOT JS Using JavaScript is only one way of implementing DHTML
Now PHP and Perl that is when you start talking about REAL langages ,plus you don't have to worry about Browser settings or Incompatibilty