Internet Explorer Developer Tools Unknown Document Mode

Question

I've been scouring the internet for some documentation on IE Developer Tools "Document mode: unknown" and have had no success thus far. I ran into this while loading a PDF export in Internet Explorer and I'm not sure if this is anything to be concerned about.

My question is, when is it okay to see "Document mode: unknown"? Is there any professional documentation of this occurrence?

Answer 1

The Document Mode tells you what HTML rendering mode the current page is being displayed with.

You'll get "unknown" in the document mode if the current page is not HTML. In your case, I guess it's a PDF document.

This is perfectly normal; it just means that the browser isn't using any of its HTML rendering modes for the document.

By contrast, you still have a browser mode. This is also normal behaviour.

The Browser Mode field remains in place because that tells you what User Agent string the browser is using. It isn't related to the actual page rendering, so it isn't affected by the fact that the page isn't HTML. The user agent string is sent with all http requests, regardless of what the file type is, so it is always present.