It’s a tradition from the fixed-width font typewriter era IIRC. Quote mark followed by period looked bad because of all the whitespace.

In the US (and Canada?).

British English puts commas and periods (full stops) outside the quotation marks unless the quotation is also a complete sentence or the punctuation is part of the quotation.

Source: https://www.unr.edu/writing-speaking-center/student-resource...

I wish we would do away with those nonsense traditions. I tend to ignore them but some correction softwares don't like it. Especially since they are incoherent between languages with no good reasons. English wants the dot in the quotes, French wants a space before colons, yada yada

Is it? I've only ever seen this phenomenon in English text, and it's definitely not a thing in Portuguese.

You're assuming a context free grammar, this is a context sensitive grammar.

Much like <b>being bold <i> being both </b> being italic </i> is a malformed CFG but a perfectly valid CSG.

Ironically it's the one advantage XML type notation has over Lisp type notation when it comes to expressiveness. It's rather unfortunate that, essentially, all XML tools assume proper nesting.