[css-transforms-2] transform-style: preserve-3d should only have effects on transformable elements #6430

As discussed in #1950 and as changed in #3750, css-transforms-2 currently says:

A computed value of preserve-3d for transform-style establishes both a stacking context and a containing block for all descendants.

I tend to think this should only happen for transformable elements, which are what transform-style's Applies to line says it applies to. It's worth noting that the "Applies To" line isn't really defined to do anything other than cause the property not to have a used value.

However, I think the intent of this "Applies to" line is that the property only have effects on transformable elements -- i.e., that it shouldn't have effects on non-replaced inlines. In particular, the effect here would be only the third place in CSS where non-replaced inlines could be a containing block for fixed-positioned elements (the other two being filter and backdrop-filter).

So I'd propose changing the wording of the above sentence to instead say:

A computed value of preserve-3d for transform-style on a transformable element establishes both a stacking context and a containing block for all descendants.

I think this still fits with the original intent of the wording which is that the effects of grouping properties not be considered in determining whether to establish a containing block or stacking context.

cc @mattwoodrow @smfr