Units & Selectors
One of the reasons CSS is so tough to master is that even apparently small decisions can sometimes have big consequences. Should you size your fonts in ems or pixels? Which selector should you use? Is there a way to avoid having to use JavaScript to accomplish what you want?
Having a good grasp of units and selectors makes all these decisions, if not simple, at least a little easier.
Units
Which CSS units do developers use?
Pseudo-Elements
Which pseudo CSS selectors do developers use?
Combinators
Which combinations of CSS selectors do developers use?
Tree & Document Structure
Which structure related CSS selectors do developers use?
Attributes
Which attributes CSS selectors do developers use?
Link & URLs
Which links & URLs related CSS selectors do developers use?
Interaction
Which interaction CSS selectors do developers use?
Form Controls
Which form related CSS selectors do developers use?