Refer to CSS 2.1 instead of CSS Box Level 3

[gsnedders]

Box Level 3 is basically abandoned and hasn't been touched much in a long time, CSS 2.1 is still what should be referenced for the visibility property.

[gsnedders]

I was going to say this needs to be in Level 1, but it's only an informative reference so the W3C Normative Reference Policy doesn't come into play here.