css_units.txt

em   - Relative to the font-size of the element (2em means 2 times the size of the current font)	
ex   - Relative to the x-height of the current font (rarely used)	
ch   - Relative to width of the "0" (zero)	
rem  - Relative to font-size of the root element	
vw   - Relative to 1% of the width of the viewport*	
vh   - Relative to 1% of the height of the viewport*	
vmin - Relative to 1% of viewport's* smaller dimension	
vmax - Relative to 1% of viewport's* larger dimension	
%    - Relative to the parent element