http://www.w3.org/TR/css3-values/

Based on http://codepen.io/anon/pen/JLdzg

400px - pixels (device pixels)
20em - relative unit
20rem - root em
4.167in - inches
10.5833cm - centimeters
105.833mm - millimeters
300pt - points
25pc - picas
40ex - x-height
40ch - based on width of zero (0) character
50vw - viewport width
50vh - viewport height
50vmin - smaller of vw or vh
50vmax - larger of vw or vh