W3 Schools

This is the top-of-class resource for learning all the core web technologies:

HTML 4
HTML 5
XML DOM
JavaScript
HTML DOM
jQuery
SQL
CSS 1,2,3
PHP
XSLT
XPath
XSL-FO
Color Picker
HTML Colors
CSS3 Browser Support

Free tutorials, reference sheets, and home of the cool "try it yourself" editor that lets you try code out right in the browser window - no editor, files, or setup required!

This is one of my favorite sources and I recommend it highly.

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