How did Leopold I gain a claim to the Spanish throne through Margarita Teresa?

Leopold I gained a claim to the Spanish throne through his first marriage in 1666 to Margarita Teresa, who was the daughter of King Philip IV of Spain and a sister of Charles II.12

Unlike her sister Marie-Thérèse, who renounced her succession rights upon marrying the French king, Margarita Teresa made no such renunciation. Furthermore, she was explicitly named in Philip IV’s will as the next heir to the Spanish throne following her brother, Charles II.1

Leopold I later sought to solidify these claims by persuading his daughter with Margarita, Maria Antonia, to bestow her rights to the Spanish succession upon him and the sons of his third marriage. While the validity of this transfer was considered dubious, it formed the basis for the immediate Habsburg claims to the Spanish inheritance during the War of the Spanish Succession.12