I always intended to start a blog properly with a post about this topic (I am the one that he refers to in the article), but I never quite got round to it... and so the full article has been sitting mostly written for about fifteen months and entirely written for about ten months. I don't understand myself. And as for the bug report which I should have filed two years ago when I first discovered the issue...

The `transform` approach is interesting; I never tried to find a workaround for it but just accepted it (again, rather than filing the necessary bug report...).

"If there isn't a bug reported for it, the bug doesn't exist."

Can someone explain the practical usage of diagonal CSS borders?

Sure: making geometric shapes and masks.

A varient of diagonal borders is currently used in http://nicolasgallagher.com/pure-css-speech-bubbles/demo/

The trick there is to use the CSS :after pseudo-element to get a triangular shape/mask, but it's a bit of a hack. Having diagonal CSS borders that were properly supported would make doing stuff like this much cleaner and easier to implement.