I was always taught that two spaces follow a period. When I got into web design, I was disturbed that HTML only acknowledges one space, but I was able to get past it because it’s a programming language. Two spaces can be ambiguous in code.
Now, a writer friend tells me that the Chicago Manual of Style recommends using only one space after a period. I like to think of myself as an open-minded, transformational grammarian, but some changes–like this one–really stick in my craw.
I wrote this entire post using single spaces, and it almost hurt to do it.