There's a good case for expanding slightly on plaintext. It's not just decorative - some usecases like Right-to-Left or lists or linked are helped by a little

There's a good case for expanding slightly on plaintext. It's not just decorative - some usecases like Right-to-Left or lists or linked are helped by a little markup, and HTML is good enough. The problem is going overboard on CSS (maybe none should be allowed) or allowing any javascript at all. I can't recall any email security issue ever which is HTML only without any CSS or javascript.