[hnlmorg]

Browsers can build upon the same base as game engines.

The issue isn't so much the technologies available (though I've often cited reasons I'm unhappy with the status quo of web technologies) but more than game engines don't need to be compatible with every game in existence. Whereas web browsers need compatibility with every website.

It's compatibility that's hard.

Another example of how compatibility is hard is looking at how operating systems support older applications. A lot of the problems Windows has is due to its commitment to backwards compatibility. Whereas Apple take a different approach and expect developers to port their applications to newer releases of macOS.