What units of measurement can be used for font size?

For font-size, you can use relative and absolute units.

Relative:

• em, relative to the parent's font size
• rem, relative to the root element's font size (html)
• %, a percentage of the parent's font size
• vw, vh, a fraction of the browser window's width or height
• vmin, vmax, a fraction of the window's smaller/larger side

Absolute:

• px, pixels
• pt, typographic points
• pc, picas
• in, inches
• cm, centimeters
• mm, millimeters

In practice, px, em, rem, and % are used most often.