The canvas.captureStream method returns a black screen on iOS but works in Safari on MacOS. You can use the official WebRTC example to replicate the bug: https://webrtc.github.io/samples/src/content/capture/canvas-video/
<rdar://problem/58946744>
Already exists as Bug 181663. But it seems that nobody at Apple likes coffee...
Duping to bug 181663. *** This bug has been marked as a duplicate of bug 181663 ***