There are a number of elements of British English that would get an American student marked wrong on an English exam, and vice versa.
This is because half the point of higher education is to master pedantry. There's a huge overlap in the cognitive equipment required to perform careful scholarship and lint-picking misplaced letters and words.
Students aren't actually marked "wrong" on their tests, despite the convention to speak about it this way. Their answers are marked "acceptable" and "unacceptable".
In an undergraduate course in computer science on an assignment devoted to algorithmic efficiency, I had a program that ran two orders of magnitude faster than the class median marked 6/10 because I didn't write my program in the mandated coding style with the mandated level of inane comments (requirements which I rejected then, and have continued to reject ever since). The professor liked Pascal and hated C. My coding style was closer to K&R and P. J. Plauger than Wirth.
Be liberal in what you accept, and conservative in what you send.
In order to be maximally conservative, one must strive for some degree of consistency. There's no way to do this without adopting some kind of norm.
There's a reason why some editors strongly prefer the Oxford comma. If you don't use it (I tend not to), there are situations where you can end up with your sentence not saying what you intended it to say.
Among those interviewed were his two ex-wives, Kris Kristofferson and Robert Duvall.
In the worst case, you can end up embroiled in a libel lawsuit. Many of the stylistic codifications accused of pedantry are similarly battle tested.
The additional social process that sometimes takes this too far is that you get a team of editors working on manuscripts from multiple authors. If every author has a different style guide, or the editors don't have a consistent reference, the group effort to achieve a consistent manuscript quickly degenerates.
Unfortunately, this often gets taken to the extreme limit, until you have obscure rulings on the picayune whose utility is obscured in the mists of time.
I learned to touch type on a manual typewriter, inserting two spaces after the sentence final punctuation mark. In the younger generation, this is portrayed as a fuddy-duddy convention. Do they even know that an advanced typesetting system sets the inter-sentence gap differently than the inter-word gap when they make this declaration?
I continue to use the double space convention when typing because it makes it easier to proof-read what I've written. My eyes are used to the double space to help me quickly navigate my sentence boundaries. And the extra space is pretty much effortless to type.
Going to the extreme of portraying the established conventions as nothing more than a bunch of "he said / she said" is complete bullshit. It's difficult to come up with a set of conventions that maximizes the conservatism (in the Postel sense) of a written text. What's the logic for coming up with your own? It's not so different than coming up with your own software license. There's a significant likelihood that what you come up with isn't legally solid, and there's a considerable burden imposed on everyone else to navigate Yet Another Vanity License. Why don't you also roll your own encryption method? It could work.
For me where it goes to far is when the standard authorities (e.g. Chicago Manual of Style) seem to forget that language standards are living standards. The underlying technology changes and the publishing demands also change. What was justifiable thirty years ago is perhaps irrelevant today.
I personally can't stand folding punctuation marks under an end-quotation mark. As far as I'm concerned, that's a matter for the layout engine, if it ends up being done at all. On the input side, it's just semantically wrong. All you get for it is a slight improvement of the visual tidiness on the printed page, at the cost (sometimes) of creating ambiguity in the reader's mind about whether the punctuation mark belongs to the quoted material, or not. Only a crazy person advocates at the same time for the Oxford comma (which averts ambiguity) and for end-quotation punctuation folding (which introduces ambiguity). Aesthetics or semantics? Make up your damn mind! (For myself, I use the Oxford comma as necessary and I make a point of being able to identify those cases.)
I'm sure most people sense that the argument in favour of standard usage as "just another style" mainly comes from people who wish to avoid effort and mastery rather than double down in the honourable spirit of Jon Postel.