[css-page-3] Specify how to handle page sizes with zero area

Filing this issue to hopefully get some clarification on two related issues that I came across recently when thinking about @page { size: ... } which I think need to be addressed in the spec, for the size descriptor in @page rules:
https://w3c.github.io/csswg-drafts/css-page-3/#page-size-prop

(1) What should be done if 0 is specified for size? (for either or both of the width & height components)

(2) Or, for nonzero page sizes -- what should be done if the size isn't larger than the specified margins for the page?

(In case 2, the naive approach would produce a sheet of paper with zero space for content, which is not great.)

Based on my testing in https://bugzilla.mozilla.org/show_bug.cgi?id=1807985 , it looks like Chromium handles both of these by falling back, effectively treating the size as auto at used value time, I think. I suggest we add that to the spec.

(I've got a bunch of testcases attached to https://bugzilla.mozilla.org/show_bug.cgi?id=1807985 as well, FWIW.)

Review the CSS Page 3 size descriptor section linked in the issue and the test cases attached to Mozilla bug 1807985. Determine the specified behavior for zero-area sizes and sizes no larger than the margins, then update the specification to document the agreed handling and verify it against the reported browser behavior.