Passed to ThumbnailPlaceholder by a card, from the dump and its rich
content, or through Thumbnail's placeholder.
optional
url: string
The dumped URL, whose hostname seeds the hue and the initial.
optional
seed: string
Seeds the hue for a file dump, which has no URL to hash.
optional
accentColor: string
The dumped page's theme-color, used as the tint when it is valid.
optional
faviconUrl: string
optional
siteName: string
Gives the initial ahead of the hostname.
optional
glyph: string
Replaces the initial, since a file dump says more with 🎬 than with "T".
optional
className: string