How to reproduce:
1. Open the attached project's Scene labeled "SampleScene"
2. Make a WebGL build or a windows IL2CPP build
3. Host the built Player
4. Open the WebGL Player in a browser
5. Inspect the canvas

Expected result: UI elements are drawn to the canvas
Actual result: UI is not rendered and multiple errors are thrown:
"A scripted object (probably UnityEngine.UIElements.ThemeStyleSheet?) has a different serialization layout when loading. (Read 56 bytes but expected 136 bytes)
Did you #ifdef UNITY_EDITOR a section of your serialized properties in any of your scripts?"

Reproducible with: 1.0.0-preview.16 (2020.3.17f1, 2021.1.18f1), 2021.2.0b9, 2022.1.0a6
Couldn't test with: 2019.4.29f1 (UI Toolkit is not available)

Note:
- Used the UXML file from 2021.2.0b7 on affected versions