I used to recommend the Guardian as the best online style guide but it’s been usurped now that fab London agency The Writer has published their style guide.
As always with The Writer, it comes with added personality and usefulness. You don’t just get a bland factual document like the Guardian’s. Ho no, that would be way too boring. To the usual stuff about hyphenation and capitalisation, The Writer adds a list of keyboard shortcuts (for commands and symbols), explanations (for example on UK vs USA spelling), and a list of proofreading marks.
Add it to your browser favourites now: https://sites.google.com/site/thewriterstyleguide/