[cssom] "Serialize a CSS declaration block" does redundant work collecting longhands

[AtkinsSJ]

Spun out from something I noticed in #12187: The "shorthand loop" of this algorithm does redundant work each loop iteration:

1. Let longhands be an array consisting of all CSS declarations in declaration block’s declarations that that are not in already serialized and have a property name that maps to one of the shorthand properties in shorthands.

It's then only used to populate current longhands with a subset of its contents:

4. Append all CSS declarations in longhands that have a property name that maps to shorthand to current longhands.

(It's also used in step 2 to reject shorthand if it doesn't have what it needs - but that could easily use current longhands instead.)

longhands itself doesn't serve any purpose as-is, it just is an extra array to allocate and add items to. I suspect the intention was as an optimization: If we only populate longhands once, before the loop, then it gives us a shorter list to iterate on each loop iteration, instead of having to look at every declaration each time. (I'm not convinced that would always be faster, fwiw, and it's unusual for a spec algorithm to attempt to optimize things.)

[cdoublev]

In my opinion, populating longhands in the shorthand loop is appropriate because most properties are not mapped by a shorthand, and even fewer are mapped by more than one shorthand.

You can populate longhands before the shorthand loop and skip doing it when shorthands is empty, but I am not convinced that would be clearer in the spec, or faster in implementations.

[AtkinsSJ]

I disagree, I think creating longhands inside the loop has no benefits over creating current longhands directly:

• It's more complicated to populate longhands matching a list of shorthands.
• You get a list of declarations that may or may not match shorthand.
• You only care about shorthand so the rest are wasted work and thrown away immediately.

Unless I'm missing something, it's only useful if done once, outside the loop.