Earlier quoted context omitted.
I don't know about that, one data point: an ES2018 feature, regex lookarounds, is still not implemented in Safari. And the JS engine is the thing that's the most compatible across browsers, nowhere near the level of incompatibility of the rendering engine for example.
Ignore stuff that was added to HTML, CSS, and JS for last 4-5 years. You'll still have a pretty solid? GUI platform, likely more capable and accessible than Qt or GTK or AWT. With the usual compiler / transpiler stack, you'll have a nice, fast-to-market, non-esoteric development environment. All without the need to ship 100MB binaries.
Not that that would fix the situation, there are still rendering inconsistencies between browsers when using stuff like margins floats and tables.