CSS units

Note how some of the units (as applied to the divs below) are relative to their parent or relative to window size. Others are absolute. When it comes to the most useful? It's rem, because it is not affected by its html parent, only what the root/html css font-size is. Change the font-size above and see.

.px { width: 100px; height: 100px;}
.pt { width: 100pt; height: 100pt;}
.in { width: 10in; height: 10in; }
.cm { width: 10cm; height: 10cm; }
.em { width: 10em; height: 10em;}
.rem {width: 10rem; height: 10rem;}
.per { width: 50%; height: 50%: }
.vwvh { width: 50vw; height: 50vh; }