As noted, there are two situations where an element will have no layout
node here:
1. The element is invisible in a way that it generates no layout node.
2. We haven't built the layout yet.
This protects against the second case, which would otherwise incorrectly
send us down the path of looking directly at the computed style.
The CSS box-shadow property takes 2-4 properties that are `<length>`s,
those being:
- offset-x
- offset-y
- blur-radius
- spread-radius
Previously these were resolved directly to concrete Lengths at parse
time, but now they will be parsed as LengthStyleValues and/or
CalculatedStyleValues and be stored that way until styles are later
resolved.
