I am using the standard webview to play YouTube videos in an iFrame.
The app / webview can play these properly on all non-amazon devices (Sony Android TV, nVidia Shield, etc). However, on Fire Stick 4K and Fire Cube (presumably all fire os devices), the video plays but I only get audio and a grey or black screen for the video.
I have setup a simple test page for you to reproduce the issue,
This page loads correctly using Silk on the Fire OS device. However, not with the webview. The application does have hardware acceleration enabled. As I said, it works properly on other Android devices.