<!DOCTYPE html>
<html class="reftest-wait">
  <iframe sandbox
          style="width: 200px"
          srcdoc="<canvas style='text-indent: 50vw; display: block'>Text</canvas>"></iframe>
  <script>
    onload = function() {
      var i = document.querySelector("iframe");
      window.w = i.offsetWidth;
      i.style.width = "400px";
      document.documentElement.className = "";
    }
  </script>
</html>