Sure but even for web apps, you have to code around the fact the people might have outdated browsers, and even with current browsers you end up having to deal with rendering quirks of each.

When you use a truly "deprecated" platform, you have don't have to worry about that; the Sega Genesis/MegaDrive hasn't been supported in any official capacity for more than 20 years, meaning that basically any emulator made in the last decade or so will be "feature complete".

(I know there are a few bits of weirdness with emulating expansion chips for Virtua Racing, but my point overall still stands).

this is about their only redeeming quality to me, ease of maintenance and compatibility. From a design standpoint with all the mix of too many technologies on shabby scaffolding it is annoying. But you know what they say "it works". I have some high hopes for web assembly to reduce some of this. or at least hide it.