Density: A raw 648px, B srcset 2x -> ~324 CSS px, C 648w + sizes:300px -> ~300px

A:
B:
C:

Broken-image fallback: D alt text only, E alt="" nothing, F no alt -> grey box

D: a description of the missing photo
E:
F:

G: cover img half-outside an overflow:hidden viewport must not bleed past the red border: