>Maybe things have changed without my knowing, at least according to those two references. And I am willing to change outdated habits. Still, I'm not sure I agree that two spaces after a period is just an outmoded idea owing to monospaced typewriters. I still think extra spacing between sentences is a good idea, to set them apart. And wouldn't a proportionally spaced font tend to actually close up the space, since a period is so narrow?

The period is narrow in Arial, but there are fonts where there's some spacing to the right of it, so with the additional 0h20 (aka single space) after it, there's just enough spacing. Funny, though, in non-proportional fonts the dot is somewhat to the left, so it comes closer to the word, thus increasing the space on the right - then they type additional 0h2020, and   there's a lot of space separating them now, eh?.

Seeing how your text doesn't look much different from mine (your two spaces vs my one), I'd just propose that we keep our habits. Neither hurts, while changing them would.