Perhaps I am being obtuse, but I fail to see what an “online” C compiler might do that a normal, “in situ” locally-resident compiler could *not* do— at least as regards the productivity and output of the user/programmer.
What has happened in practice (as nearly as I can tell), is that “online” compilers have added so-called features that have to do with “show business” and friendliness/teaching, and displays of internal workings that are educational (sort of).
However, overall this has not had much (if any) effect of increasing the efficiency and productivity of the software development process. Internet and “online-ness” are about attraction of eyeballs onto and clicks through a given website. It is okay (and perhaps even a clever niche-twist) to use and present a fundamental software tool like a compiler as a way to attract eyeballs and clicks. But, unless doing this enables development of software to be obviously better and quicker in some fashion, the bottom line is that programmer/users of online tools are serving the business purposes of the web site owners, and are not serving their own purposes any better, by using an online compiler (C language or otherwise).
Now, if someone could give us a nice, friendly Clang interpreter with all the features and power of a standard C compiler, that processed exactly the same source code as standard compiled C, that would (IMHO) do something noticeable for the productivity of C programmers…
(NOTE— this has been done. However, last I checked that product was NOT open source, and the license costs of that wonderful desideratum was prohibitively high).