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Extra space after periods is never correct with proportionally-spaced fonts, which is why all browsers remove it by default.

Two spaces after periods is a kludge invented for typewriters that had monospaced fonts and touch typing teachers need to stop teaching it in the modern era where most writing uses proportional fonts.

Curiously, it‘s also a tell-tale sign that a North-American typed the text.

Just as a space before colons or exclamation marks is a sign that someone francophone typed it.

Indeed - especially the space before colons and semi-colons. The space before exclamation marks sometimes happens in informal typing amongst Brazilians. But never the space before colons/semi-colons.

browser rendering does. You'd need white-space: pre-wrap rule to retain double spaces.

Harper will detect those errors in its next release.