Well, if such a bytecode were being standardized, someone would surely write a shim JS library to convert it to JavaScript on browsers that didn't have native s

Well, if such a bytecode were being standardized, someone would surely write a shim JS library to convert it to JavaScript on browsers that didn't have native support yet. And I think the idea of making a binary format would be more popular for a sublanguage which is essentially guaranteed to be machine-generated and inscrutable, especially given that I've seen a lot of commenters here and elsewhere have a knee-jerk reaction against the idea of using JavaScript syntax, than for HTML/CSS/JavaScript, which have a long history of being written and read manually without any (de)compilation steps, even if most big webapps are minified.