The Wikipedia article for Juan Bautista Martinez del Mazo is here.
This and the next five images are of del Mazo's painting of Margarita de Austria. The next five images are enlargements of various parts of the portrait, possible due to the high resolution of images provided by the Prado. The next five images were made from this image by cropping with Photoshop. Some spots and vertical flaws were removed with Photoshop.
This is one of the most well-known examples of Spanish court dress. This gown has all of the features - curving overskirt, expanded skirts, bertha, and a large bodice ornament crowned by her coiffure expanded on the right and balanced with feathers on the left. Her gown has no discernible false sleeves or ruff. In addition, her gown has large, slashed sleeves and ruffled cuffs.
Museo Nacional del Prado posts super-high resolution images, such as this one, here.
Keywords: 1665, del Mazo, Infanta Margarita, Empress, Habsburg family, Spanish, long straight coiffure, jeweled feathered headdress, straight neckline, full puffed sleeves, slashed sleeves, back flared cuffs, vee waistline, farthingale, saya, over-skirt, bows, earrings, jeweled bodice ornament, brooch, bracelets, rings, handkerchief