I really wouldn't call this an OS, the terms becoming a bit liberal.. theres got to be some reality in that browser javascript doesn't have nearly enough knowledge of the hardware, anything more specialized well... it just aint 'javascript' that we think of, and then theres looking at the gigantic complexities a modern OS deals with..
Sure it might be an OS in the same way as something like DOS, but calling something an OS today only inspires people to compare it to Windows, Linux, OS X.. in which case you are wayyyyyyy out of touch..
(summary; calling oneself something that may (or may not) be technically true, but certainly isn't true in a persons modern 'non-dictionary' perception of it - is lame)