Support the full set of apis for DocumentFragment
Browser extensions such as Stylus/Stylish use templates which call .firstElementChild but we can't port these to Edge because the full DocumentFragment api isn't supported right now.
These APIs are included in EdgeHTML 17 released today (30/04/2018)
This has landed in our development branches. The full set of DocumentFragment members have been implemented.
prepend/append from parentNode are covered by https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/16036408-implement-dom4-methods-prepend-append-befor
qS and qSA should already be supported
The other members are currently only supported on element. I've created a task internally for adding them to the parentNode mixin and hooking them up.