--- a/js/src/devtools/rootAnalysis/analyzeHeapWrites.js
+++ b/js/src/devtools/rootAnalysis/analyzeHeapWrites.js
@@ -21,16 +21,17 @@ function checkExternalFunction(entry)
         /^rusturl/,
         "memcmp",
         "strcmp",
         "fmod",
         "floor",
         "ceil",
         /memchr/,
         "strlen",
+        /Servo_DeclarationBlock_GetCssText/,
         // Assume that atomic accesses are threadsafe.
         /^__atomic_fetch_/,
         /^__atomic_load_/,
     ];
     if (entry.matches(whitelist))
         return;
 
     // memcpy and memset are safe if the target pointer is threadsafe.