59 very important Web Design & Usability Guidelines according to "relative importance"

Fifty-nine web design and usability guidelines had been identified by US Department of Health & Human Services as very important. These 59 usability guidelines had the score of 5 (on rating of 1 to 5 points) for the aspect of “relative importance.”

1 Use an Iterative Design Approach
2 Provide Useful Content
3 Use Black Text on Plain, High-Contrast Backgrounds
4 Ensure Visual Consistency
5 Provide Consistent Clickability Cues
6 Use Clear Category Labels
7 Ensure Usable Search Results
8 Ensure the Homepage Looks like a Homepage
9 Do Not Use Color Alone to Convey Information
10 Eliminate Horizontal Scrolling

11 Allow Simple Searches
12 Create a Positive First Impression of Your Site
13 Organize Information Clearly
14 Display Information in a Directly Usable Format
15 Detect Errors Automatically
16 Put Critical Information Near the Top of the Website
17 Do Not Display Unsolicited Windows or Graphics
18 Label Data Entry Fields Clearly
19 Use Unique and Descriptive Headings
20 Distinguish Required and Optional Data Entry Fields

21 Evaluate Websites Before and After Making Changes
22 Minimize User Data Entry
23 Format Common Items Consistently
24 Provide Feedback on Users' Location
25 Set and State Goals
26 Show All Major Options on the Homepage
27 Provide a Search Option on Each Page
28 Design for Common Browsers
29 Put Labels Close to Data Entry Fields
30 Title

31 Use an Iterative Design Approach
32 Provide Useful Content
33 Use Black Text on Plain, High-Contrast Backgrounds
34 Ensure Visual Consistency
35 Provide Consistent Clickability Cues
36 Use Clear Category Labels
37 Ensure Usable Search Results
38 Ensure the Homepage Looks like a Homepage
39 Do Not Use Color Alone to Convey Information
40 Eliminate Horizontal Scrolling

41 Allow Simple Searches
42 Create a Positive First Impression of Your Site
43 Organize Information Clearly
44 Display Information in a Directly Usable Format
45 Detect Errors Automatically
46 Put Critical Information Near the Top of the Website
47 Do Not Display Unsolicited Windows or Graphics
48 Label Data Entry Fields Clearly
49 Use Unique and Descriptive Headings
50 Distinguish Required and Optional Data Entry Fields

51 Evaluate Websites Before and After Making Changes
52 Minimize User Data Entry
53 Format Common Items Consistently
54 Provide Feedback on Users' Location
55 Set and State Goals
56 Show All Major Options on the Homepage
57 Provide a Search Option on Each Page
58 Design for Common Browsers
59 Put Labels Close to Data Entry Fields

The above 59 usability guidelines were extracted from Research-Based Web Design and Usability Guidelines.