This is mostly just a convenience shorthand to avoid running the same
boilerplate/error-checking code in the implementation of the 'width' and
'height' attributes, for the benefit of callers that need both sizes. But,
beyond that convenience: this also lets us elegantly express the case where an
image simply lacks an intrinsic width and/or height (not due to any error, but
simply because it's e.g. an SVG that has no preferred size in either or both of
its axes). This is a state we'll need to be able to detect and give special
treatment to in a forthcoming patch for bug 1935269.
Differential Revision: https://phabricator.services.mozilla.com/D248526
a1b0d56c0a