onload / onerror for <link rel="prefetch">

Hey WebAppSec'ians,

There's a question on the HTML spec that I think could use attention from
this group:

https://github.com/whatwg/html/issues/1142 (<--- hard to parse Github issue)
https://bugzilla.mozilla.org/show_bug.cgi?id=1268962 (<-- slightly clearer
Bugzilla bug)
https://bugzilla.mozilla.org/show_bug.cgi?id=1268962#c21 (<-- my analysis)

Basically, there are some web devs that want to be able to tell whether a
prefetch worked or not.  But that would create a general mechanism to probe
for the presence of resources cross-origin.

The question is: Does this cross-origin information leakage matter in
practice?  Enough to warrant doing something CORS-like just to gate the
load/error events?

Personally, this seems pretty marginal to me, given that you can already
probe for resource types that have their own tags (<img>, <script>, etc.).
But I'm open if others have concerns.

Thanks,
--Richard