Where do you want the c# code to run?
If you want it to run in the browser that is being used to render the html then I'd say that was basically not possible. Something, presumably javascript code in the html page, would have to somehow instantiate a .net clr and pass the c# code to it for execution. The clr does have a COM-based hosting interface that would allow instantiation, but even if you could call this from javascript I think that any sensible browser security settings would prevent it.
If you want the c# to run on the server supplying the html page then you should use asp.net.
EDIT
Ok, you want to run it in the browser. I'm not aware of any examples for hosting a clr in the browser process, sorry.
You can create activex/com objects in js using something like var obj=new ActiveXObject("<comclassname>");, and you might be able to create a CLR that way by instantiating one of the COM classes (maybe CLRRuntimeHost) listed on this page. You could then pass your c# code to your clr for execution. More info here and here. I'm really not sure if that would work, though. I've never used the hosting api, I just know it exists!
Seems like an interesting project to try if you are curious, but deploying this in a real environment would likely present lots of problems. Good luck!
format.exe?