Or just write it in java
Was about to say "If we only have some kind of runtime system, which could run an app exactly the same on every supported operating system without needing to port that app, that would be awesome!". :D
If you want performance, you actually wouldn't want it to be exactly the same, ironically. You'd want to include hardware specific optimizations where needed. And unless Java has changed dramatically, this would at least require implementing the core performance critical parts in another compiled language.
Well, yes. I totally agree with you on that one!
The original comment was about (mis)using the browser as some kind of platform independent runtime environment for apps.
Admittedly, at my workplace, I created a few (very simple) html/javascript based apps to get some tasks done quickly (like sorting some tables, or a little task planer tool) as it is convenient, because a browser is something you have everywhere.