Salaam alaykum brother. May Allah reward the Islamweb team for their efforts in enlightening Muslims. My question: "Can the the title 'Your Highness' and the like be used to address rulers when Highness is only applicable to Allah azzawajal?"

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

Shaykh Ibn ‘Uthaymeen   was asked about the terms: ‘Majesty’, ‘His Majesty’ and ‘His Highness’, and he answered saying: “It is permissible if the person about whom the statement is said deserves such description, and provided it is not feared that this will lead that person to pride and haughtiness.”

Moreover, the Permanent Committee of Iftaa’ answered a similar question saying: “Many names are common in wording and in general conceptual in meaning between Allaah and His Creatures, so they are attributed to Allaah with a meaning that suits Him and befits His Majesty, and they are given to a creature with a meaning that suits him and is peculiar to him. For instance, we say Allaah is Haleem (i.e. The Forbearing) and the Prophet Ibraaheem (Abraham) is also called Haleem, but the Hilm (the forbearance) of Ibraaheem is not the same as the Hilm (the forbearance) of Allaah. Also, Allaah is The Ar-Ra'oof (The Compassionate) and The Ar-Raheem (The Especially Merciful) and the Prophet, sallallaahu ‘alayhi wa sallam,  is also compassionate and raheem, but the compassion and mercy of the Prophet Muhammad, sallallaahu ‘alayhi wa sallam, are not like the Compassion and Mercy of Allaah with His Creation. Allaah is also the Owner of Majesty and Honor and is the Most Generous in an absolute manner, but every Prophet is kareeem (generous) and honorable but these qualities of every Prophet are not like those of other Prophets, and they are also not like those of Allaah, but everyone has his own above qualities that befits him.”

Therefore, it is permissible to use these common names and attributes to refer to the creatures in a manner that suits them.

Allaah Knows best.