Given that runtimes like Lua are comparable in size to a single image, and/or could be cached in web browsers, I don't think that will much of a problem. And the more that WebAssembly adds, the smaller that will get.

Obviously if we're trying to run a full JVM or CLR in WebAssembly, that will cause problems, but C# and F# both have compile-to-js targets that would likely just get faster.

Something that never really worked was cached versions of jquery (and similar) from cdn. One reason is that browsers didn't give them any particular precedence. The other was that jquery was moving quite fast compared to devs using it so as an end user i nearly never benefited from the caching. I wonder if this might work better for language run times. Maybe browsers could be provided with some manifest saying 'this site needs python 3+' or 'this site needs python 3-3.5' so that end users end up with less runtimes cached.

You can probably fit the runtime in a 10th of the size of JQuery.

You'll need most of a runtime anyway, since it provides much more than GC.