To reproduce:
1. Open any large project (user's project and builds are added in Edit)
2. Build and run it on WebGL

Expected: when loading built project, progress bar will be shown
Actual: window stays empty until Unity logo shows

Note: changing "splashScreenStyle": "Dark" to "Light" or vice versa in ProjectName.json works only for some machines
(Progress bar was shown, on empty projects)

Reproduced on 5.6.0f2, f3
Not reproducible on 5.5, 2017.1.0b1