What do you mean by it? The browser itself? In that case, everything done in the browser could theoretically be pushed down a level from user space into kernel space, potentially saving a lot of overhead, no? The only trade of I imagine is the context switch you hit from doing everything in userspace, but as far as I'm aware, even browsers have to eventually hit the kernel for at least raw input output on the network.

Why not let the OS do more and more of the complicated technical stuff, simplifying the deployment of web apps because now we don't have to know a thousand different bloated libraries, just a slowly updating set of OS APIs.