I asked the group who looks after the Touch spec.
The answer to the question is that the specification was written after the fact. It's just a reflection of the behaviour which already exists in the browsers, rather than something written with good standards and practices in mind which browsers should strive to adhere to.
In other words, Safari and the other browsers didn't add these properties (possibly an oversight, as suggested by @Oriol in his comment), and that's that.
I'm told that lobbying Apple to add new features is extremely difficult, and so since the W3C don't rate their chances of success tweaks like this are not made to these sorts of reverse-specifications.
The suggestion is to look to the future of PointerEvent
s, which inherit from MouseEvent
and so should have these offsetX
and offsetY
properties.