Flag of the Habsburg monarchy

Since the days of Rudolph of Habsburg and the 1283 Treaty of Rheinfelden, the combination of red-white-red was widely considered to be the Austrian (later also Inner Austrian) colours used by the ruling Habsburg dynasty. Black and gold later became the colours used by the Imperial House of Habsburg when they held the title of Holy Roman Emperor, as they did from the mid-15th century all the way to the dissolution of the Holy Roman Empire in 1806, and were themselves derived from the banner of the empire.

Source: Wikipedia — Flag of the Habsburg monarchy (CC BY-SA 4.0)