Is there any standard inbuilt control(like the Mediaplayer) in the new UWP SDK for Webm? Android supports it AFAIK, so shouldn´t the UWP SDK support it to port Android apps onto Windows Phone? If there are no standart controls, is there any other way to play Webm? I dont want to use a Webviev to Play Webm videos.
WinRT does not support WebM by default.
But, microsoft posted an article 'Using FFMpeg in Windows Applications'. As of this, you can use ffmpeg as MediaStreamSource.
Using FFMpeg in Windows Applications
And, FFMpeg supports WEBM. Maybe, UWP should be able to play WEBM by using FFMpeg.
(But I haven't tried yet by myself. because the build proc is complicated...)
Compile and Use FFmpeg Libraries for Windows Runtime (Windows 10 or Windows 8.1 Apps)