XHTML and CSS references

Since we receive a lot of questions concerning XHTML and CSS, I think that posting links to helpful CSS / XHTML sites would come in handy. Whenever I'm developing a site for myself or a client I always refer to these sites:

Reference:

• CSS 2
• CSS Color Map
• XHTML 1.0
• XHTML 1.1
• XHTML 1.1 (2)
• W3C Home
• Proper CSS formation
• A list Apart

Validation tools:

• W3C Validation

If you use Firefox as your web browser than I suggest that you use this extension, WebDev.