Hm, you may be lost if you don't have Rust IDE handy to jump around. Web cross reference for Rust code navigation is in development, but until that is ready, the next step is https://github.com/servo/servo/blob/master/components/layout...

parallel and sequential traverse_dom is generic traversal code which calls DomTraversal::process_preorder etc to do actual work. Here is lightly edited code:

  impl<'a, E> DomTraversal<E> for RecalcStyleAndConstructFlows<'a>
      where E: TElement,
            E::ConcreteNode: LayoutNode,
            E::FontMetricsProvider: Send,
  {
      fn process_preorder<F>(&self, traversal_data: &PerLevelTraversalData,
                             context: &mut StyleContext<E>, node: E::ConcreteNode,
                             note_child: F)
          where F: FnMut(E::ConcreteNode)
      {
          if !node.is_text_node() {
              let el = node.as_element().unwrap();
              let mut data = el.mutate_data().unwrap();
              recalc_style_at(self, traversal_data, context, el, &mut data, note_child);
          }
      }
  }