diff --git a/ports/geckolib/data.rs b/ports/geckolib/data.rs
index 1962243a310..02fdece2411 100644
--- a/ports/geckolib/data.rs
+++ b/ports/geckolib/data.rs
@@ -9,6 +9,7 @@ use num_cpus;
 use selector_impl::{Animation, SharedStyleContext, Stylist, Stylesheet};
 use std::cmp;
 use std::collections::HashMap;
+use std::env;
 use std::sync::mpsc::{channel, Receiver, Sender};
 use std::sync::{Arc, RwLock};
 use style::dom::OpaqueNode;
@@ -36,6 +37,17 @@ pub struct PerDocumentStyleData {
 
     // FIXME(bholley): This shouldn't be per-document.
     pub work_queue: WorkQueue<SharedStyleContext, WorkQueueData>,
+
+    pub num_threads: usize,
+}
+
+lazy_static! {
+    pub static ref NUM_THREADS: usize = {
+        match env::var("STYLO_THREADS").map(|s| s.parse::<usize>().expect("invalid STYLO_THREADS")) {
+            Ok(num) => num,
+            _ => cmp::max(num_cpus::get() * 3 / 4, 1),
+        }
+    };
 }
 
 impl PerDocumentStyleData {
@@ -45,7 +57,6 @@ impl PerDocumentStyleData {
         let device = Device::new(MediaType::Screen, window_size);
 
         let (new_anims_sender, new_anims_receiver) = channel();
-        let num_threads = cmp::max(num_cpus::get() * 3 / 4, 1);
 
         PerDocumentStyleData {
             stylist: Arc::new(Stylist::new(device)),
@@ -55,7 +66,8 @@ impl PerDocumentStyleData {
             new_animations_receiver: new_anims_receiver,
             running_animations: Arc::new(RwLock::new(HashMap::new())),
             expired_animations: Arc::new(RwLock::new(HashMap::new())),
-            work_queue: WorkQueue::new("StyleWorker", thread_state::LAYOUT, num_threads),
+            work_queue: WorkQueue::new("StyleWorker", thread_state::LAYOUT, *NUM_THREADS),
+            num_threads: *NUM_THREADS,
         }
     }
 
diff --git a/ports/geckolib/glue.rs b/ports/geckolib/glue.rs
index 8f76407749c..baf3b3b22ac 100644
--- a/ports/geckolib/glue.rs
+++ b/ports/geckolib/glue.rs
@@ -29,6 +29,7 @@ use style::parallel;
 use style::parser::ParserContextExtraData;
 use style::properties::{ComputedValues, PropertyDeclarationBlock};
 use style::selector_impl::{SelectorImplExt, PseudoElementCascadeType};
+use style::sequential;
 use style::stylesheets::Origin;
 use traversal::RecalcStyleOnly;
 use url::Url;
@@ -109,7 +110,12 @@ fn restyle_subtree(node: GeckoNode, raw_data: *mut RawServoStyleSet) {
     };
 
     if node.is_dirty() || node.has_dirty_descendants() {
-        parallel::traverse_dom::<GeckoNode, RecalcStyleOnly>(node, &shared_style_context, &mut per_doc_data.work_queue);
+        if per_doc_data.num_threads == 1 {
+            sequential::traverse_dom::<GeckoNode, RecalcStyleOnly>(node, &shared_style_context);
+        } else {
+            parallel::traverse_dom::<GeckoNode, RecalcStyleOnly>(node, &shared_style_context,
+                                                                 &mut per_doc_data.work_queue);
+        }
     }
 }