Why couldn't the browser run itself in a VM instead of relying on the OS to do it? There's some file size overhead, sure, but it seems like a custom VM would be

Why couldn't the browser run itself in a VM instead of relying on the OS to do it? There's some file size overhead, sure, but it seems like a custom VM would be the best way to ensure maximum compatibility, and avoids the potential for a tangle of mutual workarounds.