The SVG import's style cascade tested every <style> rule against each
element it was asked about. The import asks about the objects outside
<defs> and the like when the file contains `display`, about each shape
when the file contains `non-scaling-stroke`, and about each <text>, its
spans and its ancestors. On a file with 2,000 class rules and 100,000
rectangles, `vectorcraft-cli info` took 3.54 s with one `display`
attribute in the file and 2.03 s without it.
The rules are now filed under the id of their selector's rightmost
compound, else under its first class, else under its type. An element
is tested against the rules filed under its id, its classes and its
type, and the rules filed under none of these, in source order. It
matches the same rules in the same order and gets the same
properties. The file with the `display` attribute now takes 2.10 s.
Docs: docs/target-app-parity.md.
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