IE10 DOM and CSS Compliance Integration Boundary

Direct answer

IE10 handles standard DOM events and CSS consistently between local file testing and production HTTP/HTTPS only when the document mode stays fixed at IE10 standards. Observed differences are usually environmental, not inherent to IE10’s DOM or CSS implementation.

Confirmed facts

• IE10 standards mode supports addEventListener, removeEventListener, event.target, preventDefault(), and stopPropagation().
• Legacy attachEvent and window.event are non-standard and are not equivalent to the W3C event model.
• CSS rendering is stable within one document mode, but IE10 needs prefixes such as -ms-flexbox and lacks many modern properties.

Likely explanation

Behavioral gaps between local and production loads typically come from:

• Proxies, WAFs, or CDNs stripping or rewriting X-UA-Compatible headers.
• Intranet zones or Compatibility View lists forcing IE7, IE8, or Quirks mode.
• file:// security restrictions blocking scripts or XHR that run over HTTP/HTTPS.

Verification steps

1. Open the page locally and in production, press F12, and confirm Document Mode reports IE10 standards in both. In the console, document.documentMode should return 10.
2. Dispatch a click on a test element and compare event.target, preventDefault(), and stopPropagation() across both loads.
3. Render a reference component with -ms-flexbox, border-radius, and a media query; compare computed styles and screenshots between environments.
4. Inspect response headers for X-UA-Compatible and confirm no intermediary is forcing an older document mode.

Missing diagnostic detail

What does document.documentMode return in your local and production loads? If either value is not 10, enforce standards mode with a stable X-UA-Compatible header before changing event or CSS code.