IE9/10: Silent failure to POST a form

Spent far too long debugging an issue reported against one of my sites where a simple (but important) form submit would almost-silently fail in "IE". After downloading the massive-but-excellent IEVMs I was able to narrow it to Internet Explorer 9 and 10 (I don't care about older versions, but it would certainly affect them too).

After checking and fixing the minor HTML validity issues, replacing the submit button with an input type="submit" and various other insignificant tweaks, I actually read the error message IE was showing me:

I'd been obsessing over the way the request was being aborted so fast and was thinking there was some kind of weird cross-origin thing blocking my POST (my page is first step in a payment process - summarising the costs, and clicking the big Pay Now button caused a POST to a third-party payment processor). I'd completely failed to notice the final bullet point in the "More Information" section which mentions SSL and TLS versions. After heading into Settings -> Internet Options -> Advanced -> Security and checking "Use TLS 1.2", everything started working perfectly.

I have no idea why Microsoft would implement new versions of an important protocol but then default the support to OFF. Must be one of the backwards-compatibility hacks that (apparently) they could then finally be rid of in IE 11, where it is turned ON by default.

Hope it helps someone.