Peter Duniho wrote:

I use two spaces when typing a plain text or fixed-width font document. I
use one space for text that's formatted with a proportional-width font. (If
someone winds up displaying my two-space, plain text typing in a
proportional-width font, that's their problem ).

That's because things that can deal with proportional fonts know how
much whitespace to insert. You don't ever want to add your own
whitespace (either horizontal or vertical).