Fixes#15398
The previous commit did most of the work here of updating the algorithm
to skip shorthands if a shorthand value was an empty string. This commit
just updates animations LonghandToSerialize's implementation of
to_css_declared to write an empty string value if the list lengths
differ (and updates the test to match).
possible shorthands matches the linked spec.
Previously substep 5 attempted to serialize the complete shorthand
declaration and substep 6 skipped to the next shorthand only if the
current shorthand was not serialized, but this did not catch empty
serializations. The spec on the other hand specifically says that the
*value* should be evaluated first and if the value is empty substep 6
should skip to the next shorthand - which is what happens now.
To do this required some refactoring which mostly simplifies the code.
Specifically:
- append_declaration_value was refactored so that importance is not
required as a arg (by moving it to the end of append_serialization)
and is_overflow_with_name was removed as an arg also (initially I
refactored it elsewhere, but it turns out it's no longer required at
all - more below). With these changes, append_declaration_value can
be used within the algorithm for to_css to obtain just the value for
substep 5.
- Substeps 7 and 8 of the algorithm become explicit (they were implicit
before) by passing the value, shorthand and importance to
append_serialization.
- serialize_shorthand_to_buffer is no longer required (as the algorithm
serializes the value first instead, as per the spec.
A surprising result of this was that I could also remove a lot of code
handling the special case of the overflow properties serialization. This
is because the overflow's LonghandToCss implementation of
to_css_declared already does the right thing according to the spec - it
writes the single value if both overflow-x and -y are equal, and
writes nothing otherwise - so that the algorithm now skips that shorthand
instead rendering the longhands.
* `LonghandId` and `ShorthandId` are C-like enums
* `Atom` is used for the name of custom properties.
* `PropertyDeclarationId` is the identifier for `PropertyDeclaration`,
after parsing and shorthand expansion. (Longhand or custom property.)
* `PropertyId` represents any CSS property, e.g. in CSSOM.
(Longhand, shorthand, or custom.)
Using these instead of strings avoids some memory allocations and copies.
