[css-masking] Module interactions

   # these effects are applied after elements have been sized and positioned
   # according to the Visual formatting model from [CSS21].

Might want to mention the interaction with transforms and filters: are
things masked before or after those operations?

   # Some values of these properties result in the creation of a containing
   # block, and/or the creation of a stacking context.

When do any of these properties cause the creation of a containing block?

~fantasai