I, too, was confused for the longest time, but the secret is in the names.

En dash is the width of an “n”. Em dash is the width of an “M”, so a little wider.

> I always get en and em confused

Yeah, it’s somewhat a typography-nerdy thing :–) (gosh that looks terrible probably; edit: actually it’s surprisingly fine)

> indicating that these things are not supported

Of course, but it does sound a bit unnatural IMO. Then again, I’m going by the vibes here.