Question about making your game playable in different resolutions/dimensions Thread

I'm in the designing process of a multiplayer browser based game.
And since I'm going to code the game myself after it's planned I can allow myself to think ahead about how/best way to program some things.. which can
affect parts of my design.

Right now I'm thinking about the fact that people playing your game are going to have different screen dimensions and resolutions.
I'm thinking this is going to be quite challenging.. and therefore I'm thinking of making the game area.. where the character is jumping around etc.. to be static size. and the interface around it will be changed size.. like just making the border expand or decrease in size the smaller or bigger their screens are.

But the thing is, if I make it a static size.. then it's going to be really, really small.
Because afaik, the standard width for most websites to be built in is 960px.
And inside that area they put everything that will affect the look of the website.

Also, would it be hard to make a button that switches the game to fullscreen? It's hard to imagine how I would do with all graphics then.. just increase their size? it will make the game look weird then I would imagine.
I will as I mentioned be making this game browser based and am thinking of using html5, java, mysql, php mainly.

Just have an options panel with resolutions that people can pick to there native resolution.