What's this "Legacy Layout" they're using as a comparison? The previous iteration of Servo? Looks like some of the code is still used by both, as the curves sometimes (but not always) go up and down at the same time...
There's a good report in the Servo wiki from this year (authored by a group of Igalians) summarizing the differences between the two and why the decision was made to move forward with Layout 2020.
https://github.com/servo/servo/wiki/Servo-Layout-Engines-Rep...