Is the collapsing border model's implementation in web browsers valid?

Question

I have been trying to understand this excerpt from CSS 2.2 specification for a while with no success (the bold selection is mine):

UAs must compute an

Answer 1

The answer is "no." I love the frankness of the discussions had by the CSSWG, and the notes on the current draft of the CSS Tables 3 editors' draft tell you all you need to know about this question.

Since browsers handle this so differently, convergence cannot happen without reimplementation. …

… some combinations are not well-posed problems, so no rendering algorithm could be optimal.

Because they grew from something simple (HTML) to something very complex (HTML+CSS), the current table rendering models…used by web browsers are insane (in the sense they are buggy, not interoperable and not CSSish at all). Many usual CSS assumptions are broken, and renderings diverge widely.

(Emphasis added.)

There's much more information in the current draft, but the CSS Working Group acknowledges (1) that browser implementations are inconsistent, and (2) even their own current proposal is insufficient.