IIRC it's not just supporting legacy systems, but also things like printing (and print to PDF) need a different renderer. I'm not sure; I forgot the details.
> hat Rust was written for Servo, at least originally
Rust was initially called "The Servo Project", actually
I did a brief keynote at Mozilla's Whistler, BC all hands in 2010 proposing Servo, a new layout engine programmed in Rust. Servo was named by Dave Herman and me in honor of Tom Servo from MST3K. Rust was never "The Servo Project".
http://venge.net/graydon/talks/intro-talk-2.pdf
(also picked up on my T2 robot from the future joke and reversed it, lol). But Rust was Rust, even then. Servo was the big news, I think Rust was known within Mozilla, a bit. So we used Servo in the session name and the rest is history.