--- a/layout/style/CSSStyleSheet.cpp
+++ b/layout/style/CSSStyleSheet.cpp
@@ -1786,21 +1786,27 @@ CSSStyleSheet::SubjectSubsumesInnerPrinc
     return NS_OK;
   }
 
   // Allow access only if CORS mode is not NONE
   if (GetCORSMode() == CORS_NONE) {
     return NS_ERROR_DOM_SECURITY_ERR;
   }
 
-  // Now make sure we set the principal of our inner to the
-  // subjectPrincipal.  That means we need a unique inner, of
-  // course.  But we don't want to do that if we're not complete
-  // yet.  Luckily, all the callers of this method throw anyway if
-  // not complete, so we can just do that here too.
+  // Now make sure we set the principal of our inner to the subjectPrincipal.
+  // We do this because we're in a situation where the caller would not normally
+  // be able to access the sheet, but the sheet has opted in to being read.
+  // Unfortunately, that means it's also opted in to being _edited_, and if the
+  // caller now makes edits to the sheet we want the resulting resource loads,
+  // if any, to look as if they are coming from the caller's principal, not the
+  // original sheet principal.
+  //
+  // That means we need a unique inner, of course.  But we don't want to do that
+  // if we're not complete yet.  Luckily, all the callers of this method throw
+  // anyway if not complete, so we can just do that here too.
   if (!mInner->mComplete) {
     return NS_ERROR_DOM_INVALID_ACCESS_ERR;
   }
 
   WillDirty();
 
   mInner->mPrincipal = subjectPrincipal;