Her Wikipedia article is here. She served as Regent of Spain while her brother Felipe (II) went off to marry Mary.
From Wikimedia; some small spots throughout image removed with Photoshop and image made substantially lighter by gogm.
Juana wears a severe Spanish-style dress with a conical farthingale in solid black broken up by jewels and the white of her ruff, cap, and veil in this 1557 Coello portrait. Her dress appears to have hanging sleeves adorned with barely visible aglets.
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