what is the best resolution I can use for my game. It is a super hero action type game. it should be playable on all platform. what is in my mind is 900 something on width and 600 to 700 on height. what do you guys say? 
I would use common resolutions. For 4:3, GD defaults to 800x600, but I would bump it up to 1024x768 at least. For 16:9 widescreen, 1280x720 at minimum. Higher resolutions make it look nicer, however, it does raise the requirements for playing the end product.
If mobile devices are in your list of âall platformâ you should consider lower/common resolutions 
I did 960x620 is it cool, now half of my game is done i donât think i wanna change or should i?
It should work. Graphics chips will still be able to manage it in some way, but if it is put into fullscreen, it could scale, or it could stretch, depending on what the graphics are set to do. There may be some mobile devices that use that ratio, but I couldnât be sure. The important thing is to test it as you go along. Try to squash bugs early, so you donât have to fix on a bigger scale.
Just as an added note: I checked the ratio of the resolution you are using. Scaling the width to some of the more common sizes for computers (1024, 1280, 1980) puts the height at an odd number of pixels (661, 827, 1279), or at least thatâs what Paint.net did when I changed the width. The actual scaling may be different than what Paint.net uses.
Html5 games are popular due to it runs on mobile browsers so considering the mobile browsers and also considering the sponsorship you must develop game in width:320px, height:480px as I do
+1
Iâve made a game in 800x600, and itâs slow as hell, both on loading and ingame. And itâs a pointân click, not really an action-heavy type of game. Remember your HTML5 game depends first of the browser. And itâs ability to balance the code with the available resources.
There are so many terminals out there, that you should choose the lowest of them and scale your game to it.