When a (custom) validator is added to a link field and the field value is not valid, the UI does not show it. Only when you hover on the field a tooltip appears.
This leads to the situation that the editor cannot save the form but does not know where the problem is. (Which field fails)
If a validator fails on a text field, a red border is indicating that the field not valid. This should also work for link fields.
I also attached our custom validator (MimeTypeValidator) and the dialog definition of the respective field.