This data structure represents the various optional descriptors that can be
applied to a @font-face rule in CSS. These are used to create a [FontTemplate]
from the given font data used as the source of the @font-face rule. If values
like weight, stretch, and style are not specified they are initialized based
on the contents of the font itself.
FontDescriptor describes the parameters of a Font. It represents rendering a given font
template at a particular size, with a particular font-variant-caps applied, etc. This contrasts
with FontTemplateDescriptor in that the latter represents only the parameters inherent in the
font data (weight, stretch, etc.).