--- a/layout/tools/reftest/reftest.jsm
+++ b/layout/tools/reftest/reftest.jsm
@@ -621,16 +621,18 @@ function getStreamContent(inputStream)
 // Build the sandbox for fails-if(), etc., condition evaluation.
 function BuildConditionSandbox(aURL) {
     var sandbox = new Components.utils.Sandbox(aURL.spec);
     var xr = CC[NS_XREAPPINFO_CONTRACTID].getService(CI.nsIXULRuntime);
     var appInfo = CC[NS_XREAPPINFO_CONTRACTID].getService(CI.nsIXULAppInfo);
     sandbox.isDebugBuild = gDebug.isDebugBuild;
     var prefs = CC["@mozilla.org/preferences-service;1"].
                 getService(CI.nsIPrefBranch);
+    var env = CC["@mozilla.org/process/environment;1"].
+                getService(CI.nsIEnvironment);
 
     // xr.XPCOMABI throws exception for configurations without full ABI
     // support (mobile builds on ARM)
     var XPCOMABI = "";
     try {
         XPCOMABI = xr.XPCOMABI;
     } catch(e) {}
 
@@ -714,17 +716,19 @@ function BuildConditionSandbox(aURL) {
 
 #if MOZ_WEBRTC
     sandbox.webrtc = true;
 #else
     sandbox.webrtc = false;
 #endif
 
 #ifdef MOZ_STYLO
-    sandbox.stylo = prefs.getBoolPref("layout.css.servo.enabled", false) && !gCompareStyloToGecko;
+    sandbox.stylo =
+       (!!env.get("STYLO_FORCE_ENABLED") || prefs.getBoolPref("layout.css.servo.enabled", false)) &&
+        !gCompareStyloToGecko;
     sandbox.styloVsGecko = gCompareStyloToGecko;
 #else
     sandbox.stylo = false;
     sandbox.styloVsGecko = false;
 #endif
 
 #ifdef RELEASE_OR_BETA
     sandbox.release_or_beta = true;