Transforms and z-index (Was: [CSSWG] Minutes and Resolutions 2011-04-20)

On Apr 21, 2011, at 7:27 pm, fantasai wrote:

> Transforms
> ----------
> 
>  <glazou> http://www.w3.org/mid/AF6CAE42-3067-4B7F-9F6D-31502ECFE327@me.com
>  glazou: Next is about z-index.
>  smfr: The Transforms spec says that transformed elements act like
>        "relatively positioned element".
>  smfr: Webkit has this just create a pseudo-stacking context (like 'opacity'
>        does), but FF and maybe IE let z-index work.
>  smfr: I argued that we don't let left/top/etc apply, so z-index shouldn't
>        either.  So my proposal is that we change the spec to have transforms
>        create a pseudo-stacking context like 'opacity' instead.
>  dbaron: I don't particularly care, but I think it might be good to ask
>          authors.
>  smfr: If authors want to apply z-index, they can just actually make it
>        relpos.
>  fantasai: I think having to use relpos for z-index is confusing anyway.
>  smfr: Stacking contexts are confusing anyway.
>  sylvaing: What about opacity?  z-index doesn't apply?
>  smfr: No.
>  sylvaing: So you're just wanting to make it consistent with opacity.
>            Sounds good.
>  RESOLVED: transforms create a pseudo-stacking context, not a full one.
>            z-index doesn't apply.

I realized that was forgetting part of the story here.

A transformed element acts as a stacking context  (like one with opacity). But
it also acts as a positioning container for its descendants, so, for example, a
position:relative child's left and top are relative to its transformed ancestor in
that case, again as if the transformed element itself has position:relative.

I don't think this changes the fact that z-index should not apply to a transformed
element (just as 'left' and 'top' do not apply), but I wanted to mention it in case
someone disagrees.

Simon