Steps to reproduce:

1. Open any Project

2. Navigate to Window > UI Toolkit > UI Builder

3. Save the new USS

4. In the StyleSheets section press the "Add new selector..." text field and type ">" then Return

5. Observe errors printed

6. Then do the same with the character "%" and observe the pop-up message

Actual results: When the user inputs characters ".",">","#","-" to the Selector naming text field "Failed to calculate selector specificity: >" Errors are logged, when inputting "_" or "", the names are valid. But when inputting any other special characters, a pop-up appears that all of these characters can be inputted. 

Expected results:* We first present the user with what characters can be inputted, but then when they use them we again fall back to an Error. The Error is the same for all of the characters, so a pop-up in the same style as the first one should be used, to show users a Warning

Reproducible with versions: 2021.3.35f1, 2022.3.20f1, 2023.1.20f1, 2023.2.10f1, 2023.3.0b7

Tested on (OS): macOS Sonoma