- Add block_environment.html.haml to haml/html5 to accomodate the
  environment block used in mathematical text, e.g.,

   [env.thml]
   --
   There are infinitely many prime numbers.
   --

- Add similar block in erb/html5.

The environment block differs from the open
block only in its name - "environment" - and in that the content
is italicized, as in the theorem environment in LaTeX.  Note that
the part after "env." is arbitray: it can be "corollary", "definition",
"joke", etc.