inset() shape function missing information

[estelle]

• The information about what happens if a shape has no width or height needs to be addressed.
• how the values that exceed 100% are proportionally reduced needs to be addressed

If a pair of insets for a dimension adds up to more than 100% of that dimension, both values are proportionally reduced, so their sum is 100%. For example, the value `inset(90% 10% 60% 10%)` has a top inset of `90%` and a bottom inset of 60%. These values are reduced proportionally to `inset(60% 10% 40% 10%)`. Shapes such as this, that enclose no area and no {{cssxref("shape-margin")}}, have no effect on wrapping

Originally posted by @estelle in #33446 (comment)