I would hope it would be nice to read since one could argue Rust was designed for the purpose of building Servo. So, if you can't implement Servo nicely in Rust, it'd be a pretty bad design.
I think you've mistaken what pcwalton was trying to say. This library is interesting because (via macros) it creates a DSL that attempts to emulate how the HTML 5 spec is written, in order to more easily verify the correctness of the implementation. Note that, by dint of much of HTML being an accident of history, the HTML 5 spec is somewhat imposing; it's not going to be a cakewalk in any language, and at the same time it's bespoke enough that it would be silly to design your language to cater to the needs of the HTML 5 spec in particular. The fact that Rust can create DSLs via macros does help here, though I wouldn't recommend this approach for anything other than a similarly extreme case. In fact I'd say this library has the most extensive macro use of any production Rust code I've ever seen, it's quite atypical as far as Rust code goes.
If Rust were solely designed for the purpose of building Servo, we'd have OO - Servo quite badly wants it for the DOM - and a whole bunch of other one-off features. In practice, Rust is a language built for systems development in general with Servo having been an early testbed to make sure it's going in the right direction.
