CSS Key Concepts

• The syntax and forms of the language
• Specificity, inheritance, and the Cascade
• CSS units and values and functional notations
• Box model and margin collapse
• The containing block
• Stacking and block-formatting contexts
• Initial, computed, used, and actual values
• CSS shorthand properties
• CSS Flexible Box Layout
• CSS Grid Layout
• CSS selectors
• Media queries
• Animation

— MDN