CSS1 (Cascading Style Sheet, level 1) is the recommendation from the World Wide Web Consortium (W3C) for a standard cascading style sheet specification.

A cascading style sheet (CSS) is a style sheet derived from multiple sources with a defined order of precedence where the definitions of a style element conflict. CSS1 describes the recommended approach for Web page designers and browser developers to adhere to.

The W3C site has the CSS1 Recommendation. If you plan to use CSS1, Joachim Schwarte has compiled a useful CSS1-Properties Quick Reference Table.