(Separating out from #1432 (comment).)

Ian believes we can improve on the center-alignment rules.

In particular, he believes that the correct answer is to find the smaller of the start- and end-aligned sizes, then double that, aligning it to the abspos containing block edge that was chosen. This lets it fill out the abspos containing block as much as it can while still remaining centered on one of the "significant edges" of the staticpos containing block.

This is apparently what Edge does. Everyone else does some other "entertainingly wrong" (Ian's words) behavior.

FYI - I added two tests with this behaviour here: wpt.fyi/results/css/css-position?label=master&label=experimental&aligned&q=position-absolute-center which EdgeHTML did pass (doesn't as written as used inline-size, block-size properties).