So whom are we following, really?
I’ve always thought it’s Chicago Manual of Style. Damn, I’ve been treating this as my holy book! But now I realised that e.g. the “three letters or less” preposition lowercase rule is nowhere to be found in CMoS:
8.157 Principles ofheadline-style capitalization.
- Lowercase prepositions, regardless of length, except when they are used adverbially
or adjectivally (up in Look Up, down in Turn Down, on in The On Button,
to in Come To, etc.) or when they compose part of a Latin expression used
adjectivally or adverbially (De Facto, In Vitro, etc.).
- Lowercase the conjunctions and, but, for, or, and nor.
However it’s actually present in AP guidelines. Compare: https://titlecaseconverter.com/words-to-capitalize/?style=AP,CMOS#down
So, when in doubt, is it better to consult CMoS or AP? Or just ask around here?