I would like to start developing a MMO browser game, like Travian or Ogame, probably involving also a bit of more sophisticated graphical features such as players interacting in real time with a 2d map or something like that.

My main doubt is what kind of development tools I should use: I've a good experience with PHP and MySQL for the server side and Javascript (and jQuery) regarding the client side. Coding everything from scratch would be of course really painful so I was wondering if I should use a javascript game engine or not. Are there (possibly free) game engine you would recommend? Are they good enough to develop a big game?

Also, I saw a lot of HTML5 games popping up lately but I'm now sure if using HTML5 is a good idea or not. Would you recommend it? What are the pro and cons about using HTML5? If you'd recommend it, do you have any good links regarding game development with HTML5?

(PS: I know that HTML5 and a Javascript engine are not mutually exclusive, I just didn't know how to formulate a proper title since English is not my main language. So, please, answer addressing HTML5 and a game engine pro and cons separately)