[css-transforms-2] Individual transform properties create containing block

[ewilligers]

https://drafts.csswg.org/css-transforms-2/#individual-transforms
The individual transform properties create a stacking context, but they should also create a containing block.

For comparison, transform and perspective create both a stacking context and a containing block:

https://drafts.csswg.org/css-transforms/#transform-property
"Any computed value other than none for the transform results in the creation of both a stacking context and a containing block. The object acts as a containing block for fixed positioned descendants."

https://drafts.csswg.org/css-transforms/#perspective-property
"The use of this property with any value other than none establishes a stacking context. It also establishes a containing block (somewhat similar to position: relative), just like the transform property does."