Barking, howling mad [adjectives?]

[Listenever]

"Barking," said Uncle Vernon, "howling mad, the lot of them.”
(Harry Potter and the Sorcerer's Stone)

‘Howling’ and ‘mad’, all two are adjectives in dictionaries. What is the reason that the two adjectives are placed in a row?

 

[Andygc]

Howling is an intensifier (as is barking). You can be barking mad, completely mad, howling mad, or just ordinary mad (in which case you just say "mad").