Cascading Style Sheets
CSS1 conformance

Conformance is in regard to User Agents

Attempts to fetch all referenced style sheets and parse them according to this specification

Sorts the declarations according to the cascading order

Implements the CSS1 functionality within the constraints of the presentation medium

Core features are required in order to meet conformance

This specification also recommends, but doesn't require, that a UA:

allows the reader to specify personal style sheets

allows individual style sheets to be turned on and off