I have seriously considered blocking letter-spacing outright in user stylesheet a few types over the last four or five years, but decided to keep it to keep an eye on how badly people are doing things.

And really, I’d prefer problematic properties to have prefs whereby you can disable them outright, rather than trying to add a style on top to nullify it. Newer stuff often does, but the old stuff doesn’t. user_pref("layout.css.letter-spacing.enabled", false) would make me happy.