Also, most of the time icons should instead fall back to nothing (and should be hidden from screen readers). The main exception that one will observe in reality is a toolbar of icons only with no labels; it’s well understood in usability circles that that’s a dangerous pattern at best—text labels and icon-and-text labels both almost always simply work better.

I deem the ligature approach bad because it encourages leaving something in the document and accessibility tree that shouldn’t be there, and most commonly in a non-semantic way (e.g. a send button might end up showing something like “paper airplane”, which is terrible, or with this specific font, “Learn more →” might become “Learn more arrow_right_alt”), or with semantics that you’ll definitely break at some point (e.g. report_problem is , but if you use that for an alert icon it’ll mislead anyone that ends up reading report_problem; in short, any new semantics you desire for an icon, you need to create a new ligature for it if the user can possibly ever be exposed to it).

I just set overflow: hidden and a width of 1em (so it’s a square)

In which case the layout may survive, but I expect the "labels" that appear when the icon font fails will be cropped to their first couple of letters or so: not a great fallback in terms of helping the user understand what they're meant to be.

If they don't load the icons (whether they are image icons or font icons or svg icons), the presentation is badly busted in any case.