IE10 on Windows Phone 8: designMode Disables Scripts Globally

On IE10 for Windows Phone 8, setting document.designMode = "On" on a page has a surprising side effect: scripts stop executing in that tab permanently, even after navigating to completely different pages. The user can follow links, type URLs in the address bar, and browse normally — but JavaScript is silently disabled for the remainder of that tab’s lifetime.

function main() {
    alert("Done! Go anywhere you want using the address bar now");
    document.designMode = "On";
}

This is a denial-of-service from a web page’s perspective — a malicious site could silently cripple scripting in the victim’s tab without any visible indication. Modern banking and login pages depend on JavaScript, so a user sent to a site after this trigger would find those pages broken in ways that are hard to diagnose. Tested on Windows Phone 8 IE10.

Found during my years at Microsoft (2006–2014). These bugs were patched long ago — shared here as a historical record for learning purposes.