NOTE: The real bug here is that the ":" prefix actually does something. It should do nothing when the pseudo state is unknown (which is the case here). The reason why the selector without ":" does nothing is an unrelated problem and likely a user-side problem (will need investigation but likely the GraphView stylesheets are overriding the user one).

Reproduction steps:
1. Open the attached “IN-90844” project
2. Open the “Herve” window (Herve > Open Dialogue Editor)
3. Observe the Grid style
4. Open the “Herve/EditorResources/StyleSheets/DialogueEditorViewUSS.uss” Style Sheet
5. Remove the “:” prefix from the “GridBackground” selector
6. Observe the Grid style

Expected result: Grid style is applied without the “:” prefix
Actual result: Grid style is not visible without the “:” prefix

Reproducible with: 6000.0.32f1, 6000.1.0a9
Couldn’t test with: 2021.3.54f1, 2022.3.55f1 - “Tab” class is compilation errors

Reproducible on: Windows 10
Not reproducible on: No other environments tested