> Why can't we have a standardized generic bytecode and VMs to run it?

And why stop there! There are plenty of "languages" that currently compile into HTML and CSS (like HAML and SASS for example). If we have a common VM for scripting then why not a common VM for HTML and CSS as well. Heck, we could do this all with a single VM and a common API. For backwards compatibility, we could use a new HTML tag to enable this VM. Furthermore, as a proof of concept, you could design a plugin for all the major browsers that implements this.