While drafting #7612 (comment) I noticed that we are introducing a weird inconsistency in the Grid specification:

The static position [CSS2] of an absolutely-positioned child of a grid container is determined as if it were the sole grid item in a grid area whose edges coincide with the content edges of the grid container. However, if the grid container parent is also the generator of the absolutely positioned element’s containing block, instead use the grid area determined in § 10.1 With a Grid Container as Containing Block.

In general, an inset:auto abspos has a static position which does not change no matter where in its ancestor chain position: relative is introduced. But here we are making the application of grid positioning properties to determining the static position conditional on whether it is also the abspos containing block. See testcase

I don't know if it's Web-compatible, but I think we should not be making it conditional. It should be unconditional: either applying always to grid children’s staticpos calculations, or never applying to them. (I prefer always; it gives the author more power.)