Earlier quoted context omitted.
There was a time when JVM ran in the browsers.
Yup, via Java Applets. But they were a pain to use, that's why I intentionally wrote "seamlessly" :P
I actually enjoyed using them to get around a few browser limitations around the mid-to-late 00s, and they seemed feature/performance competitive with Flash unless what you were doing fit the media authoring model closely. But by then people were skeptical about Java and if you had to do anything to get it installed they wouldn't, and the direction was native web.