In CSS, em is the font size. It's a scalable unit, i.e. changes proportionally to the font size, as opposed to 'fixed' measures such as points. It's used typically for the indent of the first line of a paragraph or for indenting the left margin of a list or spacing between paragraphs. EX is a unit of measure that is relative to the font size. The measurement is equal to the fonts lowercase 'x', although an ex value will be defined even if the font doesn't have a lowercase 'x'.
Pixel units are relative to the screen resolution of the viewing computer.
2 Percentage Percentage is relative to the parent element's font size. The percentage value is the parent's font-size times the percentage. Corp. All Rights Reserved.