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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

From w3schools.com