--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/wptlogging.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/wptlogging.py
@@ -71,17 +71,20 @@ class LoggingWrapper(StringIO):
 
     def __init__(self, queue, prefix=None):
         StringIO.__init__(self)
         self.queue = queue
         self.prefix = prefix
 
     def write(self, data):
         if isinstance(data, str):
-            data = data.decode("utf8")
+            try:
+                data = data.decode("utf8")
+            except UnicodeDecodeError:
+                data = data.encode("string_escape").decode("ascii")
 
         if data.endswith("\n"):
             data = data[:-1]
         if data.endswith("\r"):
             data = data[:-1]
         if not data:
             return
         if self.prefix is not None: