> I am trying to download only files ending in .html
> that contain a certain string or strings
Httrack can only filter on the url, not text within the file.

-* +*.html
or
-* +mime:text/html
Will get only the html. The first, only those files that end in html exactly,
the second will get all html including other server extensions including no
extensions (site\directory)

Use your PC's search tools once you get the files