A collection of guidelines compiled by the US HHS for web usability.
The Department of Health and Human Services (HHS) has compiled a number of studies and research data into this document. The result is a number of useful guidelines to consider when writing for the web. Each guideline is rated as to importance and strength of evidence, so you - the reader - can draw your own conclusion as to how important any particular guideline may be. Guidelines include suggestions on presentation (amount of text, color choices, etc), appropriate sizes, designing for the most popular browsers while still allowing for less-popular user-agents (like screen-readers or mobile web browsers), and even minimizing load times and other "hardware" concerns.
While a little dry, this does represent a good (and well-indexed) collection of usability guidelines.
This survey of the research literature is an indispensable reference in my work. There are 209 guidelines (stress on guidelines, not do's and don'ts), each including indicators of "relative importance" and "strength of evidence" as exemplified in the book cover illustration.
Indispensable, comprehensive, if at times dated, list of heuristics with ratings for level of importance against evidence to support the heuristic (many heuristics being apparently plucked out of some web guru's arse).