Both of your suggestions are common although you shouldn’t capitalize any word but the first (e.g. Questions fréquemment posées.)

Foire aux questions is often used because it matches the English acronym but if you want something less informal, you might write:

Questions – réponses

What’s the best translation for “FAQ” in a web interface context?
I think it’s a matter of opinion, but here is mine.

Foire aux questions is commonly used because it matches the English acronym. But – in my opinion – is does not mean anything in French and it sounds a bit familiar.

From Larousse, foire in French means:

• Grand marché se tenant à des époques fixes dans un même lieu
• Fête foraine ayant une périodicité régulière
• Endroit où tout est en désordre, où l’on fait du bruit (familier)

I think we understand the term Foire aux questions because we are used to the web, but for a person who has never seen this term (an elderly person, for example), I do not think it is very understandable.

So, for a professional website, I would suggest to use Questions fréquemment posées instead. I think it is really clearer, so much for the acronym.

In my opinion, those two (widely used) translations are somehow english-driven:

• "Foire Aux Questions" matches the acronym FAQ.

• "Questions fréquemment posées" is a litteral translation of the english.

I think "Questions fréquentes", "Questions courantes" or "Questions – Réponses"
is more idiomatic, and for once there are fewer words in french.