em, rem unit, vh and vw :
em – relative to the font size of the element.
4em means 4 times the size of the current font.
If the font size is 16px then it will be equal to 1em.
so 2em will be 32px. rem – Relative to font-size of the root element vh – Relative to 1% of the height of the viewport vw – Relative to 1% of the width of the viewport