• font size in REM
• paddings and margins in EM if related to font-size and REM if not
• media queries in EM
• line-height: 1.5 (not EM, not %, just numbers)

Tools

PX, EM or REM Media Queries? | Zell Liew

Resouces

CSS units for font-size: px | em | rem

PX, EM or REM Media Queries? | Zell Liew

REM vs EM - The Great Debate | Zell Liew

Guide: EM vs REM vs PX. Which should you use?

CSS Font-Size: em vs. px vs. pt vs. percent