Hi David,
It’s a bit more than that, depending on which SC you mean.
The SC (e.g. font-family) is saying “Don’t get in the way of users changing the font”, but there are several techniques/failures needed for it.
The “!important” one is debatable, with a user-side script you can get around that, but not by appending CSS to the body (like the Stylish extension).
For font-family, relying on font-icons to relay information would be a failure, and allowing for some text-expansion within the elements would be a technique.
A test would be to use the browser (with the inspector, or a bookmarklet / extension we can suggest/provide) to over-ride the fonts and check that content & functionality is maintained.
Cheers,
-Alastair