"Most sites that insist on enforcing Chrome-only user-agents run just fine on Firefox."

I have never encountered a site that requires a Chrome user-agent string

Some sites require a string that either (a) is in a list of acceptable strings (that includes non-Chrome strings) or (b) follows a certain format

FWIW, most websites submitted to HN, including many high traffic sites, do not require a user-agent header at all

What are some examples of these alleged "Chrome-only" sites