[css-grid] subgrid vs abspos

I didn’t find anything anything in the ED that prevents an absolutely 
positioned grid item[1] from also being a subgrid[2]. Do we want to 
allow this? What does it mean in terms of layout (especially track sizing)?

And while we’re at it: what does 'position: relative' for a subgrid mean 
in terms of layout?

[1] http://dev.w3.org/csswg/css-grid/#abspos-items
[2] http://dev.w3.org/csswg/css-grid/#subgrids

-- 
Simon Sapin