Reproduction steps:
1. Create a new project
2. Switch the platform to WebGL and build
3. Host the server on localhost (for example "python3 -m http.server 25565" (read note 4 if using python))
4. Open Safari 14.1.1
5. Go Develop -> Empty Caches
6. Try opening the hosted URL (if using the example in step 3, the URL will be localhost:25565)

Expected result: the game launches
Actual result: the game never completes loading or loads after a really long time (minutes for empty project)

Reproducible with: 2018.4.36f1, 2019.4.28f1, 2020.3.13f1, 2021.1.14f1, 2021.2.0a18
Not reproducible with: 2020.2.0a19 (fix version), 2021.2.0b2, 2022.1.0a1

Notes:
1. The issue does not reproduce if hosting on a public domain using ngrok so I was not able to test this on iOS. The customer mentioned that the issue reproduces on iOS 14.6 but I was not able to verify this.
2. The issue is fixed in 2020.2.0a19
3. Sometimes the issue does not reproduce even after clearing caches (clearing them again usually helps reproduce it)
4. If hosting the server using python, you will need to disable compression in Project Settings -> Player -> Publishing Settings