Reproduction steps:
1. Open project in "VideoTest.zip"
2. Open "WebGLSafariTestScene.unity"
3. Build for WebGL
4. Host the build on an HTTP server (python -m SimpleHTTPServer <port>, there is more information in edits)
5. Open the HTTP link on your iOS device
6. Press the "Play" button

Expected result: a video starts playing
Actual result: the video does not start playing

Reproducible with: 2019.4.16f1
Could not test with:
2018.4.30f1 (precompiled assembly exception when building)
2020.1.17f1, 2020.2.0b14, because "build.loader.js:1 Uncaught ReferenceError: unityFramework is not defined at HTMLScriptElement.n.onload (build.loader.js:1)" is thrown
2021.1.0a8 because "wasm streaming compile failed: TypeError: Failed to execute 'compile' on 'WebAssembly': Incorrect response MIME type" is thrown

Devices tested:
Reproducible with:
VLNQA00310 iPad Pro 12.9 1st gen (iOS 13.4.1) (Chrome, Safari)
HELA0166 iPad 6th gen (iOS 14.2) (Safari)
Not reproducible with:
VLNQA00335, Samsung Galaxy S20+ 5G (SM-G986B), Android 10, CPU: Exynos 990, GPU: Mali-G77
OnePlus A5000, Android 10, Adreno 540, Snapdragon 835 (Chrome)
Couldn't test with:
VLNQA00204 iPhone 5C (iOS 10.3.3) (browser does not support Web Assembly)

Not reproducible with Mac & Windows (Safari, Chrome, Firefox tested)

Notes:
1. I was not able to check the developer console (iOS Chrome does not have dev tools available)
2. It might be that the video link will be expired, in that case just find any mp4 file online and put the link to it in "Video" GameObject in "VideoPlayerExtension" component