Is there a typesetting guide for English on the web somewhere, whether UK or American or both?
I’m thinking of something with rules for correct hyphenation, punctuation, dashes, commas, correct typesetting of abbreviations, numbers, time, and so on and so forth.
What you really want here is not actually available on the web: The Elements of Typographic Style, by Robert Bringhurst, is widely held to be the Typographer’s Bible.
It has been called “the finest book ever written about typography”, and it is.
Accept no substitutes.
It is by no means complete after it's long "in progress" status, but The Elements of Typographic Style Applied to the Web is a cool resource. As the name implies, it attempts to bring Robert Bringhurst's classic into the digital world.
From the author's 2005 introduction:
In order to allay some of the myths surrounding typography on the web, I have structured this website to step through Bringhurst’s working principles, explaining how to accomplish each using techniques available in HTML and CSS.
Just this year, the creator of that site decided to go open with his project. You can now fork the site on GitHub to help him out.
What you're looking for, based on the specific things you mention, is a style guide rather than a manual of typography. There are many available online. (Just search for "English style guide" and you'll get plenty.)
For British and Commonwealth English, the Oxford Style Guide has quasi-biblical authority. The style guide on Dictionary.com, although it doesn't say so explicitly, follows American conventions.
Well, there used to be a number of style guides available freely on-line --- only two seem to be left:
Though you may find the European take on this of interest:
