to hyphenate or not

No hyphen.

One source I use likes hyphenating an adverb + an adjective "to facilitate reading", but only before the noun. He is a well-liked boss. ~ Our boss is well liked. This source also says "never hyphenate an -ly adverb and an adjective, but it offers no reason for the difference. I must admit that I tend to hyphenate them "to facilitate reading" - most of my colleagues here will disagree.

It is obvious that "highly" is an adverb, so there is no need to hyphenate.

ozzourtiIt is obvious that "highly" is an adverb, so there is no need to hyphenate.

I agree. I also think it's obvious that 'well' is an adverb in contexts like 'a well known author', and yet some people insist on the hyphen (well-known). Go figure.

CJ

Attributively, hyphenate, predicatively, don't:

An out-of-date certificate
The certificate is out of date

prefixes don't take a hyphen.