Angels can fly because they can take themselves lightly.

This has been always the instinct of Christendom, and especially the instinct of Christian art.

Remember how Fra Angelico represented all his angels, not only as birds, but almost as butterflies.

Remember how the most earnest mediaeval art was full of light and fluttering draperies, of quick and capering feet.

It was the one thing that the modern Pre-raphaelites could not imitate in the real Pre-raphaelites.
Burne-Jones could never recover the deep levity of the Middle Ages.