rem css

1/*vh viewport height and vw viewport width*/
2/*example*/
3div
4{
5  display: inline-block;
6  height: 100vh; /*THis will set height equal to the height of windows*/
7  width: 100vw/*This will set width equal to width of windows*/
8}

1'em' units for the font-size property will be relative to the'font-size of the parent element'.
2'em' units on other properties other than font-size will be relative to the
3font-size of the current element. 'rem' units sizes will always be relative 
4to the font-size of the root html element.

1em -> Relative to the font-size of the element 
2(2em means 2 times the size of the current font)

1  1 rem = root size of html = 16px

1/*realative to the font-size*/ 

1p { margin: 1em; }