CSS: Cascading Style Sheets

Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written inHTML orXML (including XML dialects such asSVG,MathML orXHTML). CSS describes how elements should be rendered on screen, on paper, in speech, or on other media.

CSS is among the core languages of theopen web and is standardized across Web browsers according toW3C specifications. Previously, the development of various parts of CSS specification was done synchronously, which allowed the versioning of the latest recommendations. You might have heard about CSS1, CSS2.1, or even CSS3. There will never be a CSS3 or a CSS4; rather, everything is now just "CSS" with individual CSS modules having version numbers.

After CSS 2.1, the scope of the specification increased significantly and the progress on different CSS modules started to differ so much, that it became more effective todevelop and release recommendations separately per module. Instead of versioning the CSS specification, W3C now periodically takes a snapshot ofthe latest stable state of the CSS specification and individual modules progress. CSS modules now have version numbers, or levels, such asCSS Color Module Level 5.

Beginner's tutorials

Your first website: Styling the content: This article provides a brief tour of what CSS is and how to use it, aimed at people who are completely new to web development.
CSS styling basics: OurLearn web development section's CSS basics module teaches CSS fundamentals from the ground up.
CSS text styling: Here we look at fundamentals including setting font, boldness, italics, line and letter spacing, drop shadows, and other text features. We round off the module by looking at applying custom fonts to your page, and styling lists and links.
CSS layout: Now it's time to look at how to correctly lay out your boxes in relation to one another, and the browser viewport. This module looks at floats, positioning, other modern layout tools, and building responsive designs that will adapt to different devices, screen sizes, and resolutions.

Reference

TheCSS reference is an exhaustive reference for seasoned Web developers, describing every property and concept of CSS, including:

Thesyntax and forms of the language
Specificity,inheritance, andthe cascade
CSS selectors, includingpseudo-elements,nesting,scoping andshadow parts
CSS at-rules, includingmedia andcontainer queries
CSS values and units module, includingnumeric data types,textual data types andfunctional notations
Box model andmargin collapse
Thecontaining block
Stacking andblock-formatting contexts
Initial,computed,used, andactual values
CSS shorthand properties
CSS flexible box,multi-column andgrid layout
Animation,transitions, andtransforms

Cookbook

TheCSS layout cookbook aims to bring together recipes for common layout patterns, things you might need to implement in your sites. In addition to providing code you can use as a starting point in your projects, these recipes highlight the different ways layout specifications can be used and the choices you can make as a developer.

Tools for CSS development

You can use theW3C CSS Validation Service to check if your CSS is valid. This is an invaluable debugging tool.
Firefox Developer Tools lets you view and edit a page's live CSS via theInspector andStyle Editor tools.
TheWeb Developer extension for Firefox lets you track and edit live CSS on watched sites.