Comment by pcwalton

The author isn't saying margin collapse should go away entirely (though, having had to implement it, it sure is tempting to get rid of the feature). Rather, they're talking about the situation in which a div is empty and has top and bottom margins. In that case, the top and bottom margins of that element collapse together into one combined margin, along with any preceding and following margins from elements lexically earlier and later in the document, respectively. This is a feature most designers probably don't realize exists, and it makes margin collapsing horribly complicated to implement (and, in CSS, it's important to remember that "complicated" typically means "slow").

I believe (but could be wrong) that this "collapse-through" feature was an accidental byproduct of the original CSS 2.0 definition of "adjoining margins", and not something that the original spec authors specifically contemplated.