In order of work priority:

High Priority

Selectors

Medium Priority

Namespaces, Paged Media, Print Profile,Values and Units, Cascading and Inheritance, Text, Text Layout, Line Grid, Ruby, Generated Content for Paged Media, Backgrounds and Borders, Fonts, Basic Box Model, Multi-column Layout, Template Layout, Media Queries, Speech, Color, Basic User Interface, Scoping, Grid Positioning, Flexible Box Layout, Image Values, 2D Transformations, 3D Transformations, Transitions, Animations

Low Priority

View, Extended Box Model, Object Model, Syntax, Lists, Tables, Reader Media Type, Positioning, Generated and Replaced Content, Line Layout, Hyperlink Presentation, Style Attribute Syntax, Math, Presentation Levels, Aural Style Sheets, TV Profile, Behavioral Extensions