The Inspector should provide a Cog Menu > Reset to default command for all properties.
This command will reset the property to its default value in the JavaFX API.
Note that this may be different from the value set at the time the object was added from the Library to the Content. Objects often come from the Library pre-initialised with "convenient" values which are not the API default value.
E.g. When a Button is inserted from the Library, its text property has text="Button".
Using Cog Menu > Reset to default will set text="" (empty string) which is the default value in the API.
This will usually cause the property to be omitted from the FXML document.

[already implemented -- this entry created only for completeness]