Major websites for standards and specifications
Schedule of Doctypes
Cascading Style Sheets, Level 2; detailed descriptions of css elements
Cascading Style Sheets, Levels 2 and 3; snapshot of css elements
Cascading Style Sheets, Selectors Level 3
HTML Syntax
HTML 4 Specification
HTML 4.01; Detailed descriptions and codes for HTML elements
HTML5
XHTML 1.0; Definition of XHTML
HTML 4.01 / XHTML 1.0 Tags Reference
Tutorials
HTML Dog - detailed, with beginners, intermediate and advanced sections
W3 Schools tutorials - detailed, with HTML and XHTML sections (useful as a general tutorial but beware of errors as listed in w3fools)
CSS HTML Tutorial.com - XHTML
CSS tutorial - basic*
Tutorial - Learn to code HTML & CSS
tizag.co - HTML tutorial - intermediate*
* The coding is generally HTML (not always correct for XHTML) but the sites are still useful for the basic coding principles.
Validators
HTML/XHTML markup validator
WDG HTML markup validator
CSS stylesheet validator
Accessibility validator
Readability Test
Broken links check
Broken links check
JavaScript Verifier
JavaScript Validator
Spam Detector
Searches for characteristics that search engines could consider spam
Character codes
Special HTML Characters (to eliminate validation errors with characters like £ and to show code as text to avoid browser activating symbol as code)
Code converter for special HTML Characters (to show code as text to avoid browser activating symbol as code)
Browser-safe fonts
Common fonts to all versions of Windows & Mac equivalents
Website Performance
Website Performance Tool and Web Page speed analysis
Browser Tester
See how your page looks on other browsers
See how your page looks on other browsers
Browser Statistics
Browsers statistics, popularity and screen resolution
Colors
HTML colors
Named HTML colors eg Azure
Color Converter; Hexadecimal<->RGB Dec
Color wheel; needs javascript or Active X with Binary and script behaviors enabled
Color Scheme Chooser
Color picker; copy colors from any site
Crops colors from any image
Photo software
Irfanview free image editing software
Forums for help with coding
The Web Design Forum (mainly UK members)
Killersites (mainly USA members)
Web Design Forums (USA & UK members)
Cyber Tech Help Support Forums (mainly USA members)
Web Design Forum (mainly UK members)