diff --git a/components/style/gecko/media_queries.rs b/components/style/gecko/media_queries.rs
index 1c34762f286..771f4570b47 100644
--- a/components/style/gecko/media_queries.rs
+++ b/components/style/gecko/media_queries.rs
@@ -64,6 +64,27 @@ pub struct Device {
     used_viewport_size: AtomicBool,
 }
 
+impl fmt::Debug for Device {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use nsstring::nsCString;
+
+        let mut doc_uri = nsCString::new();
+        unsafe {
+            let doc =
+                &*self.pres_context().mDocument.raw::<structs::nsIDocument>();
+
+            bindings::Gecko_nsIURI_Debug(
+                doc.mDocumentURI.raw::<structs::nsIURI>(),
+                &mut doc_uri,
+            )
+        };
+
+        f.debug_struct("Device")
+            .field("document_url", &doc_uri)
+            .finish()
+    }
+}
+
 unsafe impl Sync for Device {}
 unsafe impl Send for Device {}
 
diff --git a/components/style/media_queries/media_list.rs b/components/style/media_queries/media_list.rs
index 122f18a0d5e..a5604b6dc19 100644
--- a/components/style/media_queries/media_list.rs
+++ b/components/style/media_queries/media_list.rs
@@ -14,8 +14,8 @@ use parser::ParserContext;
 use super::{Device, MediaQuery, Qualifier};
 
 /// A type that encapsulates a media query list.
-#[css(comma)]
-#[derive(Clone, Debug, MallocSizeOf, ToCss)]
+#[css(comma, derive_debug)]
+#[derive(Clone, MallocSizeOf, ToCss)]
 pub struct MediaList {
     /// The list of media queries.
     #[css(iterable)]
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index e777921e668..aa5f77d0db4 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -75,6 +75,10 @@ impl UserAgentCascadeDataCache {
         Self { entries: vec![] }
     }
 
+    fn len(&self) -> usize {
+        self.entries.len()
+    }
+
     // FIXME(emilio): This may need to be keyed on quirks-mode too, though there
     // aren't class / id selectors on those sheets, usually, so it's probably
     // ok...
@@ -90,6 +94,7 @@ impl UserAgentCascadeDataCache {
         S: StylesheetInDocument + ToMediaListKey + PartialEq + 'static,
     {
         let mut key = EffectiveMediaQueryResults::new();
+        debug!("UserAgentCascadeDataCache::lookup({:?})", device);
         for sheet in sheets.clone() {
             CascadeData::collect_applicable_media_query_results_into(device, sheet, guard, &mut key)
         }
@@ -105,6 +110,8 @@ impl UserAgentCascadeDataCache {
             precomputed_pseudo_element_decls: PrecomputedPseudoElementDeclarations::default(),
         };
 
+        debug!("> Picking the slow path");
+
         for sheet in sheets {
             new_data.cascade_data.add_stylesheet(
                 device,
@@ -117,7 +124,6 @@ impl UserAgentCascadeDataCache {
         }
 
         let new_data = Arc::new(new_data);
-
         self.entries.push(new_data.clone());
         Ok(new_data)
     }
@@ -244,8 +250,8 @@ impl DocumentCascadeData {
                 let origin_sheets = flusher.origin_sheets(Origin::UserAgent);
                 let ua_cascade_data =
                     ua_cache.lookup(origin_sheets, device, quirks_mode, guards.ua_or_user)?;
-
                 ua_cache.expire_unused();
+                debug!("User agent data cache size {:?}", ua_cache.len());
                 self.user_agent = ua_cascade_data;
             }
         }
@@ -1085,7 +1091,7 @@ impl Stylist {
         guards: &StylesheetGuards,
         device: &Device,
     ) -> OriginSet {
-        debug!("Stylist::media_features_change_changed_style");
+        debug!("Stylist::media_features_change_changed_style {:?}", device);
 
         let mut origins = OriginSet::empty();
         let stylesheets = self.stylesheets.iter();
@@ -2145,16 +2151,19 @@ impl CascadeData {
             return;
         }
 
+        debug!(" + {:?}", stylesheet);
         results.saw_effective(stylesheet);
 
         for rule in stylesheet.effective_rules(device, guard) {
             match *rule {
                 CssRule::Import(ref lock) => {
                     let import_rule = lock.read_with(guard);
+                    debug!(" + {:?}", import_rule.stylesheet.media(guard));
                     results.saw_effective(import_rule);
                 },
                 CssRule::Media(ref lock) => {
                     let media_rule = lock.read_with(guard);
+                    debug!(" + {:?}", media_rule.media_queries.read_with(guard));
                     results.saw_effective(media_rule);
                 },
                 _ => {},
@@ -2346,8 +2355,10 @@ impl CascadeData {
 
         if effective_now != effective_then {
             debug!(
-                " > Stylesheet changed -> {}, {}",
-                effective_then, effective_now
+                " > Stylesheet {:?} changed -> {}, {}",
+                stylesheet.media(guard),
+                effective_then,
+                effective_now
             );
             return false;
         }
@@ -2382,8 +2393,10 @@ impl CascadeData {
                         .was_effective(import_rule);
                     if effective_now != effective_then {
                         debug!(
-                            " > @import rule changed {} -> {}",
-                            effective_then, effective_now
+                            " > @import rule {:?} changed {} -> {}",
+                            import_rule.stylesheet.media(guard),
+                            effective_then,
+                            effective_now
                         );
                         return false;
                     }
@@ -2401,8 +2414,10 @@ impl CascadeData {
 
                     if effective_now != effective_then {
                         debug!(
-                            " > @media rule changed {} -> {}",
-                            effective_then, effective_now
+                            " > @media rule {:?} changed {} -> {}",
+                            mq,
+                            effective_then,
+                            effective_now
                         );
                         return false;
                     }
diff --git a/components/style_derive/to_css.rs b/components/style_derive/to_css.rs
index 471cf37b6de..83177f3c89f 100644
--- a/components/style_derive/to_css.rs
+++ b/components/style_derive/to_css.rs
@@ -231,6 +231,8 @@ pub struct CssInputAttrs {
 #[derive(Default, FromVariant)]
 pub struct CssVariantAttrs {
     pub function: Option<Override<String>>,
+    // Here because structs variants are also their whole type definition.
+    pub derive_debug: bool,
     pub comma: bool,
     pub dimension: bool,
     pub keyword: Option<String>,