DOM says to. Worth a try?
This should land after bug 773780 to avoid bitrot. I'll still write the patch now while I have time, and update it later.
Created attachment 671362 [details] [diff] [review] Patch The diffstat made me an immediate convert: 42 files changed, 1 insertions(+), 3484 deletions(-) Granted that it's almost all test files -- but still, they're horribly ugly tests and I'm delighted to be rid of them. A few more test changes might be needed. Try: <https://tbpl.mozilla.org/?tree=Try&rev=3f35951f7d8c> The one line inserted is because of js/xpconnect/crashtests/468552-1.html. Bug 468552 happened to use isSupported to test for an XPConnect bug -- I switched it to replaceChild instead, on the theory that it should be testing the same basic bug.
For the moment perhaps the SVG mochitest should stay but use hasFeature instead and only test SVG features. That would still allow you to get rid of isSupported wouldn't it?
Given that even removing navigator.taintEnabled() (Bug 679971) caused major problems, I'd be very careful removing this stuff. We need to have at least some data whether it is being used. So, please add first some telemetry probes.
Comment on attachment 671362 [details] [diff] [review] Patch Rev iid, please. ;) Seems fine with that.
Created attachment 724904 [details] [diff] [review] Patch, rebased I had to rewrite the patch to apply on top of the current tree, because IsSupported implementations have moved around a bunch. It builds locally and passes some tests I tried. I also adjusted the SVG test to use hasFeature instead of deleting it. The basic idea is identical to the previous patch. Try: https://tbpl.mozilla.org/?tree=Try&rev=4c0a12849b8f
Comment on attachment 724904 [details] [diff] [review] Patch, rebased r=me especially if other UAs already don't support this...
Nope, we're breaking new ground here. https://hg.mozilla.org/integration/mozilla-inbound/rev/0e7006217d7d
Mentioned on https://developer.mozilla.org/en-US/docs/Firefox_22_for_developers and put an obsolete header on https://developer.mozilla.org/en-US/docs/DOM/Node.isSupported.
I've added this bug to the compatibility doc. Please correct the info if wrong. https://developer.mozilla.org/en-US/docs/Site_Compatibility_for_Firefox_22