Something that makes color icons harder to support is the presence of dark modes and sometimes choice of themes within light mode and dark mode. Where a single set of glyphic icons they can simply be tinted appropriately, but with full color icons you’ll need at minimum a base set for light mode and secondary subset with problematic icons adjusted to work under dark mode. It’s easy to solve from a technical perspective (in CSS there’s media queries, on Apple OSes Xcode has built in variant handling for images) but there’s still the extra overhead of creating the required resources.

That's true, but most sites don't support dark mode anyways, and it's still not clear that dark mode will be widely adopted web designers. In any case, you're right there would just be a second font file for dark mode, so at least a user wouldn't have to download both.

In any case, as of two months ago Chrome still explicitly has no plans to implement SVG in OpenType (despite support in every other major browser, and the request being 3 years old), so color icon fonts don't appear to be coming to the web as a whole anytime soon. :( [1]

https://bugs.chromium.org/p/chromium/issues/detail?id=306078

We're working on duo-tone icons, and we have to find the best solution for this problem. If you any ideas please let me know.

Make Apple will introduce it and call it breakthrough ;)

Emojis are at least colourful and shaded. Maybe if the set of them increases as the rate they are currently we can have them as icons in our UIs.

And as the sibling post pointed out it’s all Unicode.

And then every other company will tease them about it before following suit.