Parsing SVG documents into useful layout specifications

In a previous post, I discussed using SVG documents to specify layouts in a Unity app. More recently, I started delving into that subject and laid out a list of things to cover:

1. How to make the SVG easy to interpret visually.
2. How to convert the SVG into a set of rules.
3. How to select the right rules given a query string.
4. Generating an SVG that explains what went wrong when a test fails.

Among the many things I deferred was how to actually parse the SVG into a set of layout constraints.

Both #2 and #3 are pretty simple and they are strongly related, so I'll handle them in this text.