Well said – mostly – especially the line length thing, which is so often left unconsidered.
The above, however, I’m afraid is simply incorrect. Double spaces between sentences date back to the time of typewriters and monospaced characters. At this time a double space did clarify confusion.
Unfortunately, it carried though as a fad into secretarial colleges for a while even after typewriters were no longer used and double spaces were not needed. Thus, every MS Word documents seemed to use it and it became the de facto norm.
In reality, with proportional fonts, it is unnecessary and, in fact, a hindrance to clear communication. The designer of the font will have considered this, unless it is one of the millions of poorly home-made fonts out there. Any extra visual space required between sentences is provided by the full stop itself, anyway
Double spaces create holes in the text which interrupt the flow, in the same way overly long or short line lengths interrupt flow.
Find some double-spaced text and squint at it, all those holes become apparent, like narrow measure, justified, un-hyphenated text.
In short, typographically speaking, it is an abhorrent anathema!
[Try it here, thankfully, the software strips it out]
Whilst we are in full-on type geekery mode, a couple of other atrocities which should be avoided at all costs – and if I had my way would be capital offences – are:
The use of straight quotes instead of typographic quote marks. It’s just lazy and ugly. Also misuse of possessives, but that is drifting into grammar, rather than typography – though equally irksome.
Using hyphens instead of en and em rules. Again lazy. They are there for a reason.
Finally, if you do need wider spaces – and there are legitimate uses – use an en space (or even an em space, on very rare occasions).
Sorry, if this comes across in any way as sanctimonious, but I’m type-obsessive, it goes with the territory!