Reproduction steps:
1. Open the attached project ("case_1248848-Sortie.zip")
2. Open the repro scene ("Demo")
3. Enter Play Mode and inspect the graphics inside Game window
4. Switch build target to WebGL
5. Make a WebGL build and run it
6. Compare the graphics in the player with graphics inside the Editor

Expected result: Build resolution matches Editor resolution
Actual result: Build graphics quality is noticeably worse than in the Editor

Reproducible with: 2019.3.0a11, 2019.4.0f1, 2020.1.0a4
Not reproducible with: 2018.4.23f1, 2019.3.0a10, 2020.1.0a5, 2020.2.0a14

Tested with: Chrome (83.0.4103.97), Firefox (77.0.1), Edge (83.0.478.45)

Note: Maximizing build fixes the issue (graphics remains being fixed after minimizing, too)