I’m using UI Builder 1.0.0-preview.11 and UI Toolkit 1.0.0-preview.13 (in combination with Unity 2020.1.16f1). Unfortunately I can’t set rich text properties in UI Builder, it says “Rich text properties can be made accessible by installing the latest UI Toolkit package”. But I’m already using the latest UI Toolkit package 
I did try to delete the Library folder, but that didn’t help. Did I miss something? Are rich text properties not yet supported by UI Builder?
This has been updated on the patch notes. You need 2021.1 with com.unity.ui installed, which isn’t compatible right now, so its a no go till they fix that.
@Chris-Trueman Thanks! Please bear with me, but I can’t find that info on the patch notes? It just states that 2020.1.15f1 or newer is required and the latest UI Toolkit version?
Here are the patch notes (thread):
https://discussions.unity.com/t/775534/15
Not all features are available in all versions of Unity. You’ll see specific (202x.y+) tags beside each feature in the release notes that requires a minimum Unity version.
And yes, Rich Text is only available in 2021.1 with com.unity.ui installed.
Thanks @uDamian 