Question: what are some css rules?
Answer: Colors. A pet peeve of mine is an over-abundance of color values in a web project. ... 
Typography. All properties that affect or are affected by the font should be declared once together. ... 
Spacing. On a content-first site, spacing should complement copy. ... 
Grid. ... 
Text alignment. ... 
Outlines. ... 
Focus & Hover. ... 
Opacity.