The offsetWidth of a span element is 0.

offsetWidth obtained by script is 0. 

For example, this html page prints 0 to the console.

``` html
<!DOCTYPE html>
<html lang="en">
<body>
  <div id="main">
    <span id="span1">Test</span>
  </div>
  <script>
    console.log(document.getElementById("span1").offsetWidth);
  </script>
</body>
</html>
```

I used Servo commit 9415252 on Ubuntu 16.04.
