[editing] Definition of "Invisible elements" is vague (#68)

The current definition:
> All elements that after the application of CSS are invisible, either due to CSS properties or because the type of element would not have been rendered even without CSS.

is too vague. For example, does an element with `visibility: hidden` considered as invisible for this purpose?  How about `visibility: collapse`.  We need a precise definition for this.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/editing/issues/68