Introductory Level
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CSS Getting Started
MDN
This tutorial introduces you to Cascading Style Sheets (CSS). It guides you through the basic features of CSS with practical examples that you can try for yourself on your own computer.
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CSS Basics
Dev.Opera
What CSS is, how to apply it to HTML, and what basic CSS syntax looks like.
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CSS Selector Classes
Wikiversity
What are classes in CSS?
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External CSS
Wikiversity
Using CSS from an external style sheet.
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Adding a Touch of Style
W3C
A brief beginner's guide to styling web pages with CSS.
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CSS From the Ground Up
Google Code University
Video tutorial on styling pages with CSS.
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Common CSS Questions
MDN
Common questions and answers for beginners.
Intermediate Level
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CSS Reference
MDN
Complete reference to CSS, with details on support by Firefox and other browsers.
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CSS Challenges
Wikiversity
Flex your CSS skills, and see where you need more practice.
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Intermediate CSS Concepts
HTML.net
Grouping, pseudo-classes, and more.
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CSS Positioning 101
A List Apart
Using positioning for standards-compliant, table-free layout.
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Progressive Enhancement with CSS
A List Apart
Integrate progressive enhancement into your web pages with CSS.
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Fluid Grids
A List Apart
Design layouts that fluidly resize with the browser window, while still using a typographic grid.
Advanced Level
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CSS3 in Under 5 Minutes (CSS3)
Addy Osmani
A quick introduction to some of the core features introduced in CSS3.
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Using CSS Transforms (CSS3)
MDN
Apply rotation, skewing, scaling, and translation using CSS.
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CSS Transitions (CSS3)
MDN
CSS transitions, part of the draft CSS3 specification, provide a way to animate changes to CSS properties, instead of having the changes take effect instantly.
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Understanding CSS3 Transitions (CSS3)
A List Apart
Start using CSS3 transitions by carefully choosing the situations in which to use them.
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Quick Guide to Implement Web Fonts with @font-face (CSS3)
HTML5 Rocks
The @font-face feature from CSS3 allows you to use custom typefaces on the web in an accessible, manipulable, and scalable way.
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This site discusses CSS3, latest, still-evolving version of CSS. Many CSS3 features are supported in various ways by modern browsers, but many users still have older browsers and so can't take advantage.