[css-masking-1] Clarify effect of mask-image: none

[johannesodland]

Although mask-image: none intuitively reads as not masking the element, it is somewhat unclear from the spec text.

In a previous version of the spec, it was explicitly stated that the element is not masked:

A value of none counts as an image layer but does not mask the element.

However, in later versions this has been changed to the following:

A value of none counts as a transparent black image layer.

This text by itself makes it seem like the element is made transparent, as masking by a transparent black image would have the same effect as setting opacity: 0. Only by reading the Mask Image Rendering Model you find out that masking is only applied when mask-image is other than none.

Could we clarify that a value of none does not mask the element, either by adding a note or adding it to the spec text?
The same applies to mask-border-source where it's clarified that an empty or failing image will not mask the element, but a similar definition for none is missing.

[ydaniv]

Looks like an error. Should be opaque white image layer.

cc @tabatkins