width=600 (should default height to 300, same aspect ratio of 2:1)
height=300 (should default width to 600, same aspect ratio of 2:1)
width=50 (should default height to 25, same aspect ratio of 2:1)
HAVE METADATA
height=150 (should adjust width to ~360px, matching source aspect ratio
width=300 (should adjust height to ~125px, matching source aspect ratio)
*Chrome breaks when you use the same source twice, but works for this test otherwise
NOTES
Safari on iOS appears to ignore the 2:1 aspect ratio and instead fall back to the default 300px or 150px specific measurements. e.g. the second example appears to be 600x150.