I tend to wrinkle my nose at Strunk & White's The Elements of Style, mostly because every time I have needed a style guide in my life, TEoS has failed me and I've had to go to other sources. (I also have an annoying aversion to popular things, I don't know why. Am I jealous of a 70-year-old English style guide?) But TEoS is online, if you like.

I prefer The Chicago Manual of Style. It is not available online, despite appearances: If you register for an account (free), you can search the 15th edition (the latest) of the guide, but the search results only give you section headings, you are expected to refer to your printed copy for the text.