Since you decided to dedicate your first ever HN post to that inquiry, I'll give you my personal reasoning.
Both black and white are very commonly lower-case. For example in the Bloomberg article, they refer several times to "black person's" and "black." I consider it incorrect to upper-case a racial identifier if you're not going to upper-case them all. I alternatively could upper-case Black and White, however I also consider that wrong, and borderline stupid (again, my personal opinion).
If I were referring to a specific national heritage, I would upper-case. Vietnamese American. Nigerian American. German American. And so on.
I also would not upper-case "a Tall American" in description of someone. A person that is an asian American is not necessarily from Asia, asian is their race. A person is not from "black" or "white" either. It's as ridiculous to upper-case asian in this use, as it would be to upper-case Brown Eyed American or Green Eyed American or "the Freckled American."
American is a proper noun in this case. White and asian (or should I rephrase it as "Asian and white?"), are used as descriptive adjectives here, also. If it were rephrased as "Whites and Asians, from America," then it would be capitalized because all three are now proper nouns.
Proper adjectives are for British English only. Even so, I don't see them often nor do I know of any current English grammar texts or thought leaders that recognize proper adjectives.
For the British English claim, I have no source nor can I find one. It's what I was taught long ago and that the Fowler brothers use it implicitly in all of their series.
However, I do have a rebuttal to your style guides. They are only that: descriptivist style guides. They clash with one another and are nothing more than another dialect of English. Whereas Loweth's English Grammar, the recognized beginners in English prescriptivism standard, does not recognize proper adjectives.
> I think it should be White Asian, do you say white americans?
We do say "White Americans", meaning Americans who are "white", which means people whose ancestors were Europeans (including Russia), or from Arabia, Persia, Afghanistan, North Africa, or some (but not all) other Muslim areas.
Then "Asian" includes everyone else whose ancestors were from Asia: China, Japan, India, Vietnam, Korea, Indonesia, etc.
When combined with nationality or geographical location, race is said first, so "White Asian" would mean someone living in Asia whose ancestors were European, most likely, though it's not a phrase I've heard.
I am not great with grammar, however I think its always supposed to be capitalized. that being said, i dont read too much into it in a hn post. Its sort of random for me, depends how tired my fingers are to leave the home keys
English grammar generally follows the rule that proper nouns and derivations thereof are capitalized. So the proper way to write it would be white, black, Asian, Hispanic (also Latino/Latina), Native American (or Indian or Amerindian), Pacific Islander, etc. Note the difference between native American and Native American.
Both black and white are very commonly lower-case. For example in the Bloomberg article, they refer several times to "black person's" and "black." I consider it incorrect to upper-case a racial identifier if you're not going to upper-case them all. I alternatively could upper-case Black and White, however I also consider that wrong, and borderline stupid (again, my personal opinion).
If I were referring to a specific national heritage, I would upper-case. Vietnamese American. Nigerian American. German American. And so on.
I also would not upper-case "a Tall American" in description of someone. A person that is an asian American is not necessarily from Asia, asian is their race. A person is not from "black" or "white" either. It's as ridiculous to upper-case asian in this use, as it would be to upper-case Brown Eyed American or Green Eyed American or "the Freckled American."