https://drafts.fxtf.org/geometry/#DOMQuad

Let left be the minimum of point 1’s x coordinate, point 2’s x coordinate, point 3’s x coordinate and point 4’s x coordinate.

See http://www.w3.org/mid/CAGN7qDCS4U=aa2bVec1OCUVSMECAgUDwZT5fSj8HS1Ue6AJozw@mail.gmail.com from @smfr
cc @cabanier