Back in the day the browser was a document browser - like a read-only, network-enabled version of Microsoft Word. It's goals have changed immeasurably, but the technology largely hasn't - at least not at the conceptual level. We use it to make applications that are much more like actual applications than they are documents. The slow march of the browser is to doing "anything" a native app can - we now have geolocation APIs, graphics card access, "threading", native drag-and-drop interop, etc., etc.

By OS, I meant it more in the way the JVM is an "OS" ("virtual machine" is too vague): it can do almost anything the actual OS can, provides platform-agnostic drop-ins for many native toolkits (e.g. GUI), and for a tremendous amount of the workload, developers needn't care needn't care what platform they're on. Now the JVM has its flaws for sure, but at least it knew its purpose before it grew up, the result being it is highly performant (less the boot time) and flexible.

If you need a document browser, sure you can build in a little macro language with only one kind of number, but if you need a VM a la the JVM you'd be well advised to build something better. My point is that we're heading somewhat blindly towards the latter by making a Frankenstein out of the former, and we really could do with pulling over and looking at the map for a bit.