The spec is a bit awkward here: A few algorithms create an "empty"
SourceSet, and then assign its source-size value a few steps later, so
we have a temporary state with no length. In order to avoid complicating
the types with Optional, I've chosen to just assign it to 0px.
Previously we used `auto`, but `auto` is not a valid value here - it is
used inside the "parse a sizes attribute" algorithm, but that always
returns an actual length (or calc).
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