--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -940,16 +940,18 @@ nsPresContext::UpdateAfterPreferencesCha
   mDeviceContext->FlushFontCache();
 
   nsChangeHint hint = nsChangeHint(0);
 
   if (mPrefChangePendingNeedsReflow) {
     NS_UpdateHint(hint, NS_STYLE_HINT_REFLOW);
   }
 
+  // Preferences require rerunning selector matching because we rebuild
+  // the pref style sheet for some preference changes.
   RebuildAllStyleData(hint, eRestyle_Subtree);
 }
 
 nsresult
 nsPresContext::Init(nsDeviceContext* aDeviceContext)
 {
   NS_ASSERTION(!mInitialized, "attempt to reinit pres context");
   NS_ENSURE_ARG(aDeviceContext);