All text should be expressed in EMs / REMs
Use percentages for Horizontal measurements
Consider using ems/rems for margins & padding if you want them to increase and decrease in proportion to the font-size
Consider using percentages for left and right margins and padding so they change in proportion to the width of the containing element
Use pixels (absolute measurement) for height/margins/paddings if you do not want those measurements to change relative to screen size
These rules are not absolute but having a standard approach speeds up development