Earlier quoted context omitted.
It is not a mostly solved problem. The people behind Slack, Spotify, and so on have actually commented on these threads explaining the exact line of reasoning. This stuff isn't limited to just CSS, and it makes total business sense to avoid it.
I don't consider Slack and Spotify to be good at making applications (despite their popularity). These are the least efficient chat app and least efficient music app ever, in a multi-decade history of chat and music apps, which didn't have the overflowing abundance of convenience and productivity offered by Electron. (This is like the obviously silly quote "we lose money on every sale, but we'll make it up in volume!…
I dunno, Discord might be gunning for that prize.