Not sure what model but its running 9.3.5.
Ah, right, there lies the problem, I think.
I’ve got an iPad 3, which must be five or six years old, in fact, March 2012, just checked, and mine has the same version of iOS as yours, the older machines, like anything with an OS, just can’t cope with later versions.
My phone is an iPhone 6+, running the latest version, 10.2.1, and I can double-tap and zoom on that, with no problem.
I believe the issue is that the recent update to the site is no longer backwards compatible with older OS, for whatever reason, probably something to do with voodoo, human sacrifice or fluffy kittens, but I’m sure that’s where the problem lies, nothing to do with Apple, any more than those having issues with different flavours of Android can lay the blame at Google’s door.
My pad will be replaced very soon with a new Air, it’s getting far too slow, now, and it’s bloody heavy to hold for extended periods, I’d suggest it might well be worth keeping an eye on what Apple announce at the end of the month, as there may well be a bunch of new Pads, including Minis, and there’s a suggestion that there may be one the same size as the regular version, but with an almost edge-to-edge screen.