For example, in Blink, <blockquote> is

custom-blockquote {
  display: block;
  -webkit-margin-before: 1__qem;
  -webkit-margin-after: 1em;
  -webkit-margin-start: 40px;
  -webkit-margin-end: 40px;
}

As @tabatkins kindly explained, -webkit-margin-* are "logical" versions of the "physical" margin-{top,right,bottom,left} properties. This allows blockquotes to work better in different writing modes.

Notably, the HTML spec does not use these, so I guess the spec is wrong there. Although, it seems possible that if it wasn't for the __qem quirk, we'd be able to just use top/right/bottom/left, since in this case top = bottom and left = right?

There is a very-rough-sketch of standard versions of these properties in "CSS Logical Properties". It has been described as "half-copypasted and half-inconsistent (you can decide which half for each part)." ;)