Probably alert the user that it can't be parsed (like browsers currently do for XHTML). Maybe do something like firefox does when it encounters a self signed SS

Probably alert the user that it can't be parsed (like browsers currently do for XHTML). Maybe do something like firefox does when it encounters a self signed SSL cert - allow the user to accept it - but make that choice explicit and painful. If most parsers actually did this, I doubt people would carelessly violate standards anymore. Of course, it's a bit of a prisoners dilemma to get everyone to actually do this. The one guy who doesn't will likely become immensely more popular than his competitors ... Oh well, it's nice to dream.