A project stylesheet accumulates dozens of one-line rules that set a margin on one element, and each of them is a name somebody has to invent and remember.
<div class="mt-3 mb-0 px-2"> <!-- margin-top: 1rem, etc -->
<div class="my-4 mx-auto">
<div class="p-0">
<!-- and the scale, which is configurable -->
<!-- 0 = 0, 1 = .25rem, 2 = .5rem, 3 = 1rem, 4 = 1.5rem, 5 = 3rem -->The direction letters are t b l r x y, and x and y are the ones worth learning first because they halve the class count. Breakpoint variants exist — mt-md-4 — which is where this stops being a shortcut and starts being a layout system. The obvious objection is that it puts presentation in the markup, and the honest answer is that it does, and that the alternative was a stylesheet nobody could delete from safely.