It's never been clear to me what "web standards" actually cause all these difficulties in the first place. A lot of the foundational stuff hasn't actually changed all
that much.
As an experiment, I downloaded Opera 12.16 from 2013 (the last Presto-based version), and it works better than you'd might expect. The biggest issue is that lots of https requests fail because it only supports outdated versions (it does support TLS 1.1 and 1.2 in the settings, disabled by default, but enabling that doesn't seem to have much effect).
There are a few things that don't render correct: cnn.com because of incomplete flexbox support, as well as some stuff their CSS minimizer does that Opera doesn't seem to like. GitHub doesn't really work correct mostly because CSS variables aren't recognized (it does render mostly okay, just without colours and such), and the JS doesn't work as Opera doesn't support const.
I couldn't test stuff like gmail, fastmail, slack, etc. due to the TLS issues, and various other sites I tested all run in to JS problems because of const, arrow functions, and similar small issues. It's certainly not usable, but overall, it's not bad for an 8 year old browser with a long-dead rendering engine.
Maybe I'll run a proxy to fix the TLS issues and filter out some of the basic JS issues and see what happens then.
I will say, it's slow. This is also an issue with fairly simple sites, like HN, old.reddit.com, lobste.rs, etc. Not sure what's up with that, I think it's related to network requests, as it significantly speeds up when stuff's cached.
Clearly it needs significant work to be useful, but from the look of things I expect that actually, it would be an entirely doable project by a not-too-large dev team to bring Presto up to at least Safari-level in a reasonable amount of time.