<test>

Units

px: A pixel. Use for setting specific sizes.

em: Width of "m" character in the element's font size. Use for scaling spacing, like in formatted text.

rem: Width of "m" character in the root font size. Use for non-scaling spacing, like in layouts.

Margin

Used for spacing elements.

https://www.smashingmagazine.com/2019/07/margins-in-css