Stacking order + 'z-index' spec

Filing this issue to track that we need a stacking/painting order spec. Topics should include (note this list has been edited substantially since the issue was initially filed):

• stacking order (for painting and hit testing), with key additions from https://www.w3.org/TR/CSS2/zindex.html that should include:
  • column rules
  • all the effects of the exported terminology described below
  • Adjust z-index description to handle grid/flex which automatically apply it not just to positioned items.
• general painting model
• general hit testing model
• terminology that other specs can hook into to to describe:
  • properties that establish new stacking context
  • properties that establish containing blocks for absolute and fixed positioned elements (or just absolute, if needed)
  • properties that establish an isolated group (already defined to be equivalent to stacking context)
• #2717
• https://github.com/w3c/csswg-drafts/issues/9139#issuecomment-1672101834
• Maybe: rasterization and what a "raster pixel" is, for defining things such as border snapping and other effects. See discussion in #7287.
• Maybe: general pixel snapping model for CSS rendering (generally snapping of edges after layout, but for some properties snapping of sizes before layout)

(Feel free to edit this description to add more tracking items, or comment below and I'll edit if you don't have permissions.)

Start by reviewing the CSS2 z-index specification and the checklist in this issue, including the linked issues and discussions. Define the scope for stacking order, painting, hit testing, exported terminology, and any rasterization or pixel-snapping work before drafting. Done means the agreed topics are specified and the listed tracking items are addressed.