i have a intranet application. We are using IE11, the application is running in IE8-mode. This is specified via the x-ua-compatible-Meta-Tag.
We are showing external applications in iframes. These applications needs to run in IE5. They add the x-ua-compatible-Tag automatically, when the mode is IE8 or lower, but the Internet Explorer sends the IE-UserAgent in request header for the frames.
Any idea how to fix that? I was searching for the whole day...
I have bad news for you. What you are trying to achieve is not possible.
IE shows all frames/iframes within a single page using the same browser mode. It cannot display different modes across different frames or iframes. It simply isn't an option.
The closest you can possibly get to what you want is to open the quirks-mode page in a new tab or browser window rather than an iframe.
While I'm answering, I would also implore you to find a way to get rid of the requirement for IE5 quirks mode. Even IE8 mode is going to be problematic for you in the near short term. Microsoft's Edge browser does not support these modes, so when they finally drop support for IE11 your site will simply not work on new systems. This isn't a long-term problem: Microsoft's stated goal is to move forward with the Edge browser and drop support for IE as soon as practical.