I think an online JCS would be quite useful. Like everyone else said, collaborations would be made simpler and on a larger scale.

WebJCS

I fixed a lot stuff, added a bunch of tilesets (about 400 I think), and WebJCS is now WAY much faster because it calculates what should be displayed and only draws that (layer view). Still, layer properties, parallax view, animation tiles and the undo function are still non-functional. What I will implant next are flipped tiles, layer properties and the B button, I hope

I have made an Web App out of it, which is why you see that cool tab in Chrome in the image below, and WebJCS will appear on the New Tab page of Chrome. See Help -> About JCS for more details. Later, when Google makes Web App Store official, I will make a Web App with offline functionality, which means you "install" JCS in Chrome!

At the moment, you can't use this "for real" yet, because it's still very limited (you can't change width, copy + paste, events...), but you can use this when you don't have access to JCS.exe, or on your cellphone/smartphone (yes, it works, but not very well in Opera Mobile ).

I will implant collaborative editing when I have completed the normal JCS functions of WebJCS. Also, I'm thinking of storing levels in the browser, not export to J2L all the time.

Should/Could this be moved to the JCS forum/Further game development forum?

I implanted support of Parallax View, so you can see how it (almost) would look like in-game, just like in real JCS. It has some bugs with layer tiling outside the level (left and top sides), but I guess you don't use that area much anyways
Works best in Google Chrome!

Here, have a clickable screenshot:

You can "disable" parallax scrolling by minimize it's window, and you will see it gets blank. The bigger the view is, the slower it goes

Progress report?

Here is a status report, as requested:

I have updated WebJCS with event support!
It's not fully working, you can't grab/put events, some event parameters are buggy, event difficulty not implanted, illuminate surroundings don't work yet.
I'll fix those later.

Screenshot:

You can also flip tiles by pressing F.
Warning, don't overwrite your original J2L files with WebJCS generated ones, to be on the safe side.

What about making Animations works?