Always carry a document (or medical passport) stating that you have HHT. HHT is a relatively rare disease, which is not easily recognized. It is therefore wise to carry a document (or medical passport) in case you have a car accident or are taken to a hospital unconscious.
The document should state that you have HHT, if possible the type (HHT-1, HHT-2) and also where to obtain more information. It is, of course, also important to state your blood group.