What is an X-UA-Compatible header?

Recommends which rendering/compatibility mode Internet Explorer should use for the page.

X-UA-Compatible was Internet Explorer-specific, telling old versions of IE which rendering mode to use for a page to avoid it falling back into a legacy compatibility mode. With IE now fully retired, this header has essentially no practical effect on today's web and only shows up on websites that haven't touched their header configuration in a very long time.

Common use case

Historically forced Internet Explorer 8 and later into their latest available "Edge" rendering mode, preventing the browser from silently falling back into a legacy compatibility view that broke modern CSS and JavaScript.

Example

X-UA-Compatible: IE=edge

History

Introduced by Microsoft alongside Internet Explorer 8 in 2009 to manage its notoriously inconsistent rendering-mode behaviour.

Deprecated / legacy status

Microsoft officially retired Internet Explorer in June 2022 in favour of Edge, which doesn't use this header's compatibility-mode system at all - it survives today only as a vestigial artefact on sites that haven't cleaned up old boilerplate.

Did you know?

It's a strong tell that a site's boilerplate hasn't been touched in a very long time - virtually no template or framework has included it by default for years.