So let's take a look at the truth behind these popular myths.
Two spaces were introduced with the typewriter because all the gaps in monospaced fonts make single spaces unclear or aesthetically bad. Proportional fonts don't need it.This is probably the most popular argument, and yet it is completely incorrect. First, wide sentence spacing was standard practice for hundreds of years before typewriters were invented. The first newspapers printed in America used it. It was used not only in printing but was even standard in handwriting (the Declaration of Independence was written that way). During this period of course, proportional fonts were the standard; monospaced fonts were virtually unheard of before the typewriter. When the typewriter came along, people put extra space between sentences because that's what they were already doing everywhere else.
The trend towards the same spacing for words and sentences didn't take place in typesetting until sixty years after the invention of the typewriter. Clearly, blaming wide sentence spacing on the typewriter is incorrect. The best one can do is blame the typewriter for preserving what was abandoned in the printing industry.
Second, yes monospaced fonts look airy and could even be described as having gaps. But the actual spaces are enormous too. Compared to proportional fonts, spaces see the biggest increase in size of any character. And the period character in a monospaced font is just as wide, creating a lot of visual space when used with just a single space. Because of the air over and around the period, and the very wide space, the only logical argument someone might make about this is the opposite of the myth: typewriter fonts had less need for the extra space compared to proportional fonts.
The Chicago Manual of Style says so.This manual, and most others that people reference are industry standards. They aren't lawbooks or grammar rules, or even really "Miss Manners" guides. They are a reference for people who work for that particular company, on how their employer wants them to perform their duties. The only people that must follow the Chicago Manual of Style are people who work for the University of Chicago Press.
Of course, anyone is free to follow them, but keep in mind that the choices made by some particular publisher might have as much or more to do with streamlining printing costs as they do with achieving the height of aesthetics or the most ideal reading experience.
Personally, I've never found cost-cutting or conforming with big business to be very stylish choices.
HTML eliminates extra spaces automatically.This has nothing to do with typography, and everything to do with how the web was designed and how it grew. HTML was built on top of SGML, and that's where it's inherited it's syntax of angle-bracketed <tags>. The way SGML works, the tags describe any meta-information about a document, which includes layout information. The stuff between the tags is the actual content. And in the content, a space's only purpose is to separate words from each other. One space or ten do that job exactly the same, and this is why spaces in HTML are collapsed.
Sentence spacing (or any other kind of spacing for that matter) is not content, it is layout. So why can't you adjust the layout of sentence spacing? In early conceptions of the World Wide Web, HTML was meant as a very simple document type that could be used to access all other document types. Even in the beginning there was resistance to adding new tags or behaviors. There was a version of the XMosaic web browser which did in fact try to detect sentences based on periods, and add extra space between them. However it was deemed to be unreliable, and unneeded. This was not an effort to conform to typography. In fact one argument for no extra space was that "writers of HTML really shouldn't need to know the fineries of typography" and quipped that letting HTML do that would be as silly as letting it do kerning. Of course that was a long time ago, and now modern HTML and CSS standards include some options for pair kerning and range kerning.
At present, I'm hoping to introduce new standards into HTML to improve it's ability to provide convenient CSS-controlled sentence spacing.
Modern computer fonts adjust their spacing automatically.Sorry there is nothing magical about fonts. Much like the typewriter argument, this one is pure fabrication. The only thing I can think of is that this refers to the pair-kerning that is built into many modern computer fonts. This tells the font that when certain letters are next to each other to adjust their spacing. This was commonly done because some letters look more airy next to each other ("eo") and others look more crowded ("dl").
However it is absolutely not common practice for fonts to set pair kerning for a period and a space. Nor should they. A period can be used in an abbreviation followed by a space, and we definitely wouldn't want to confuse this spacing with sentence spacing. Also, some people choose to use spaces between initials, e.g. "A. J. Foyt", and while using a space there is an arguable practice, I doubt anyone would argue that those spaces need to be wider.
Aside from the fonts, some software packages might automatically detect and format sentences. This is not typically on by default in modern word processors, probably because it's hard for computers to get it right by themselves. Consider this answer to the question "who's going?": "You and I. J. Smith will also go." Is that one sentence or two? In fact, if software needs to detect sentences for whatever reason, it will have a much easier time accomplishing this if users are in the habit of using two spaces between sentences.
Wide sentence spacing creates rivers of whitespace in the text.
A "river" appears when spaces on a few lines in a row all line up and create a visual effect that draws the eye. This has always been considered unsightly, but did not become a hot topic of conversation in printing until the invention of hot-metal typesetting. In hand-setting, typesetters could adjust the spacing in lines such that spaces didn't line up as much. In lines that were very "open" (didn't have much text) it was common practice to add spaces between letters in a word, which reduced the gaps between words, and hence reduced problems with rivers.
The most popular typesetting machines after the invention of hot-metal typesetting were Linotype machines. Letter spacing on these machines was impractical. Because of this, the Linotype was very prone to rivers. It should be no surprise then that Linotype pushed the world towards narrower fonts, narrower word spacing, and eliminating extra space between sentences, because all of these adjustments helped to reduce the appearance of the Linotype's inevitable rivers.
Two spaces, or one?Almost every article out there focuses on "how many" spaces there should be between sentences. This is the wrong issue entirely. Typographically, we should be talking about "how much" space we use. "How many" spaces is irrelevant in typography, just as it was irrelevant in the pre-typewriter era, where the same physical gap in text might be made from several different combinations of physical spaces and quads.
The only place where the question of "how many" is relevant is during the actual act of typing, and this is where the only clear argument is to press that space bar twice. Regardless of your aesthetic choices on how much space should go between sentences, the typewriter's convention is incredibly useful for helping computers figure out where those sentences are.
Admittedly there's plenty of software that doesn't provide good handling for sentences, and simply sticks in two actual space characters with no way to override it. But if your software doesn't do what it should, demand that the software be changed. Don't demand that everyone in the world adjust their behavior to conform to your broken software.