2 files changed, 40 insertions, 0 deletions
diff --git a/testing/web-platform/tests/common/reftest-wait.js b/testing/web-platform/tests/common/reftest-wait.js
new file mode 100644
index 0000000000..64fe9bfd7f
--- /dev/null
+++ b/testing/web-platform/tests/common/reftest-wait.js
@@ -0,0 +1,39 @@
+/**
+ * Remove the `reftest-wait` class on the document element.
+ * The reftest runner will wait with taking a screenshot while
+ * this class is present.
+ *
+ * See https://web-platform-tests.org/writing-tests/reftests.html#controlling-when-comparison-occurs
+ */
+function takeScreenshot() {
+    document.documentElement.classList.remove("reftest-wait");
+}
+
+/**
+ * Call `takeScreenshot()` after a delay of at least |timeout| milliseconds.
+ * @param {number} timeout - milliseconds
+ */
+function takeScreenshotDelayed(timeout) {
+    setTimeout(function() {
+        takeScreenshot();
+    }, timeout);
+}
+
+/**
+ * Ensure that a precondition is met before waiting for a screenshot.
+ * @param {bool} condition - Fail the test if this evaluates to false
+ * @param {string} msg - Error message to write to the screenshot
+ */
+function failIfNot(condition, msg) {
+  const fail = () => {
+    (document.body || document.documentElement).textContent = `Precondition Failed: ${msg}`;
+    takeScreenshot();
+  };
+  if (!condition) {
+    if (document.readyState == "interactive") {
+      fail();
+    } else {
+      document.addEventListener("DOMContentLoaded", fail, false);
+    }
+  }
+}
diff --git a/testing/web-platform/tests/common/reftest-wait.js.headers b/testing/web-platform/tests/common/reftest-wait.js.headers
new file mode 100644
index 0000000000..6805c323df
--- /dev/null
+++ b/testing/web-platform/tests/common/reftest-wait.js.headers
@@ -0,0 +1 @@
+Content-Type: text/javascript; charset=utf-8