One or two spaces between sentences?

"single spacing is the norm in all professional typography, on the web, and in the vast majority of email"

That's most likely due to the fact that unless someone inserts a hard-space, most whitespace is compressed, so I'm not sure what "on the web" means. Did the author actually check the HTML source?

I've been using two spaces after a sentence full-stop forever, and I think it helps differentiate between the use of periods within sentences, like for abbreviations, and the end of the line. Unfortunately, this doesn't come through with HTML though. The additional space also helps me to tell the end of the sentence easier, in the same way that indenting a paragraph or a blank line between paragraphs help communicate the flow.

I have recently been finding myself going back and removing, what I do acknowledge as extraneous, spaces when the input is limited. I like to blame twitter for this.

never! i shall fight you on the beaches... ah FFS meh.