Please read William Stein‘s blog post about the online LaTeX editor he created for the Sagemath Cloud. I’ve tried it out and it is pretty incredible, especially considering the fact that when I wrote my first post about the Sagemath Cloud two weeks ago, there wasn’t anything like this.
Of course, since the online latex editor is new there will be some bugs, etc. For me (and maybe because of me or my local setup) not all features work perfectly for some of the files in the Stacks project. But all the basic stuff works. So I would use it if I needed to do some edits and was using a laptop for example. (I am just too used to my own setup on my desktop at home/work to consider using an online editor when I am in my office.)
I strongly recommend you get an account on the Sagemath Cloud and play with it.