I beg to differ, the basic yet the most fundamental aspect of an operating system is your never ever supposed to feel it. Actually, speaking radically nobody really uses an operating system, we use programs on our computers.
The OS itself should help these programs run while itself being as unobtrusive to the cause as possible. What the operating system does is initialises hardware, allocates resources to the programs, helps locate the files in the disk and help these programs to connect to the net.
Also the most primitive definition of the kernel (the core of the OS) is that it helps interact with the hardware thereby making it easy for programmers to write programs.
The so-called web OS doesn't fulfill any of these, so while its justifyable to call it maybe something like a desktop shell such as GNOME/KDE that can be run from a browser, calling it an Operating System is just ridiculous.
As an Aside, have a look at this,
*validator.w3.org/check?uri=*eyeos.org/demo