Credit: Steve Traynor
A bazillion years ago in Internet time (aka 1995), Brendan Eich, Marc Andreessen, and the rest of Netscape looked at the World Wide Web and saw a sparsely tagged world of static documents -- a computational desert where a programmer's seed could find no purchase.
Technically, the browser doesn't offer what we've come to expect from the traditional OS. Purists will complain: Does the browser team spend any time worrying about the gnarly tangle of device drivers? Does the browser keep the file system clean and uncorrupted? Does the browser juggle multiple threads of differing priorities and help them share the same processor cores in a way that might be considered fair? The OS guys take one look at Chrome and laugh because that browser just punts, splitting itself into a different process for every Web page, letting the OS layer do the work.
Despite these very legitimate plaints from OS geniuses, the browser is the dominant layer, the one nexus for software, the one switchboard where all power lies. It needs from the operating system a rectangle to draw the Web page, a bit of storage space, and a TCP/IP feed. It does everything else in a cross-platform way that is, when all is considered, relatively free of bugs and other issues.