The Elements of Typographic Style Applied to the Web
webtypography.net
webtypography.net
There should be between about 45 and 75 characters in a single line. If there are not the user should at least be able to adjust the number of characters in a line.
Don’t justify without hyphenation. Never justify monospaced typefaces.
Use a single space between sentences.
Don’t letterspace lower case.
Pick an appropriate line height. The default is nearly always too small, especially for body text. Increase it but don’t break it.
Use the non-breaking space for mathematical and numerical expressions (like 4 cm).
Use hyphens, dashes and quotes correctly. (Not on the site and not a common mistake in the past but today it is everywhere.)