How can I use the compatibility mode in IE11 like I had it in IE10?
Under Emulation I find compatibility for documentmodus. But is this realy the old compatibility mode from IE10? Most important for me is the Feature-Set for javascript.
Under Emulation I find compatibility for documentmodus.
Yep, that's right. That's what you're looking for.
But is this realy the old compatibility mode from IE10?
Yes. This is exactly the same feature as the compatibility mode that was available in earlier IE versions.
Most important for me is the Feature-Set for javascript.
I'm not really sure what you mean by this, but the document modes available in IE11 are exactly the same as the ones available in IE10, so whatever you were doing in IE10 should be the same in IE11.
It's worth pointing out that the use of compatibility mode is discouraged in most cases (MS originally planned to drop it from IE11, and only brought it back at the last minute).
If you're using compat mode for testing that your site works with older IE versions, it is not a good solution for this, because the various compatibility modes are not 100% compatible with real IE versions. (this has always been the case; nothing new here with IE11).
For testing purposes, it is better to use real copies of each IE version that you need to support.