See https://web.archive.org/web/20171218122807/http://www.heracl... for a great article on the history of wider spaces between sentences — how there were two traditions of typography and things started to shift towards equal spaces in the 1930s–1960s (i.e. inter-sentence space being the same as the inter-word space). I'm even holding on my lap right now a book published in 1957 by a reputed publisher (Parkinson's Law — Riverside Press / Houghton Mifflin, 24th printing) that uses wider spaces between sentences. As do of course all of Knuth's books (e.g. TAOCP Volume 4A from 2011).
For some reason, the other typographical tradition (which I suspect owes its ascendancy to William Morris in some way) is dominant today (which is fine), but the way they've nearly completely rewritten history (e.g. blaming “double” spaces on typewriters) is mystifying.
I don't strictly oppose this typographical tradition; my native tongue has many widths of spaces (in particular between a measurement and the unit there should be a slightly narrower space) and I'm used to it.
What I dislike strongly is this idea that one should alter the source manuscript to emulate these typographical conventions. It makes no sense to replace a wider sentence space with two regular spaces. They are in no way equivalent!
> It makes no sense to replace a wider sentence space with two regular spaces.
Using a double space is a simple way of telling the computer that you want a wider space, because otherwise it can't tell that you mean the end of a sentence.
Three hyphens is a lot. I would have said it was more common to just use one hyphen for what should properly be an en-dash (which most people don't use) and two hyphens for an em-dash with or without spaces to separate from the surrounding words.
I can't recall ever seeing three hyphens used to signify an em dash in a document published on the net. (I added the word "published" to account for the possibility of the existence of a markup language I am unaware of that employs your three-hyphen convention.) I am fastidious enough about usage that I probably would've noticed and remembered.
In my experience, its always two hyphens (or space, hyphen, hyphen, space).
For some reason, the other typographical tradition (which I suspect owes its ascendancy to William Morris in some way) is dominant today (which is fine), but the way they've nearly completely rewritten history (e.g. blaming “double” spaces on typewriters) is mystifying.
The current version of the relevant Wikipedia article is here: https://en.wikipedia.org/w/index.php?title=History_of_senten...