Ah yes, parallelism (locking etc) part is entirely handled in generic Rust library (Rayon in this case), and not specific to Stylo at all. Here is where Stylo meets Rayon, https://github.com/servo/servo/blob/master/components/style/...

  //! Implements parallel traversal over the DOM tree.
  //!
  //! This traversal is based on Rayon, and therefore its safety is largely
  //! verified by the type system.

  /// A parallel top-down DOM traversal.
  ///
  /// This algorithm traverses the DOM in a breadth-first, top-down manner. The
  /// goals are:
  /// * Never process a child before its parent (since child style depends on
  ///   parent style). If this were to happen, the styling algorithm would panic.
  /// * Prioritize discovering nodes as quickly as possible to maximize
  ///   opportunities for parallelism.  But this needs to be weighed against
  ///   styling cousins on a single thread to improve sharing.
  /// * Style all the children of a given node (i.e. all sibling nodes) on
  ///   a single thread (with an upper bound to handle nodes with an
  ///   abnormally large number of children). This is important because we use
  ///   a thread-local cache to share styles between siblings.