MIT-licensed HTML5 template and CSS framework built for responsive design based on a 12-column grid. The boilerplate contains no JavaScript, just HTML, CSS and example site icons (favicon & iOS).
A MIT-licensed template for HTML5 websites that includes a lot of CSS and JS tweaks and optimizations. It provides support for JQuery, Google Analytics, old browsers, etc.
The official CSS page at the W3 Consortium.
The "Learn & Use" page is crammed full of links to tutorials, articles, and events. The majority of resources are in English, but there appear to be a decent number in other languages as well.
This specification defines Cascading Style Sheets, level 2 revision 1 (CSS 2.1). CSS 2.1 is a style sheet language that allows authors and users to attach style (e.g., fonts and spacing) to structured documents (e.g., HTML documents and XML applications). By separating the presentation style of documents from the content of documents, CSS 2.1 simplifies Web authoring and site maintenance. CSS 2.1 builds on CSS2 [CSS2] which builds on CSS1 [CSS1]. It supports media-specific style sheets so that authors may tailor the presentation of their documents to visual browsers, aural devices, printers, braille devices, handheld devices, etc. It also supports content positioning, table layout, features for internationalization and some properties related to user interface.