Where do tabindex="0" HTML elements end up in the tabbing order?

tabindex assignments are handled the following way (for elements that support the tabindex attribute):

Positive numbers (1,2,3...32767) are handled in tab order.
0 is handled in source order (the order it appears in the DOM)
-1 is ignored during tabbing but is focusable.

This information is taken from : http://www.w3.org/TR/html401/interact/forms.html#adef-tabindex

The HTML specification states:

Elements that have identical tabindex values should be navigated in the order they appear in the character stream.

It's a bit more complicated than Alan Haggai Alavi's answer.

After parsing, IE8 and Opera do as the HTML4 spec says. Firefox and Chrome however use DOM order. This matters with malformed markup like this.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
  <head>
    <title>Test case 1</title>
  </head>
  <body>
    <form>
      <table>
        <tr><td><input id="first" value="first in the character stream" tabindex="0"></td></tr>
        <div><input id="second" value="second in the character stream" tabindex="0"></div>
      </table>
    <form>
  </body>
</html>

You might well argue that with malformed mark-up all bets are off anyway, so what about JavaScript?

Consider this case:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
  <head>
    <title>Test case 2</title>
    <script type="text/javascript">
      moveFirst = function()
      {
        var f = document.getElementById("first");
        f.parentNode.removeChild(f);
        document.body.appendChild(f);
      }
    </script>
  </head>
  <body>
    <form>
      <table>
        <tr><td><input id="first" value="first in the character stream" tabindex="0"></td></tr>
        <tr><td><div><input id="second" value="second in the character stream" tabindex="0"></div></td></tr>
      </table>
    <form>
    <div onclick="moveFirst()">move</div>
  </body>
</html>

In this case, when a user clicks on "move", IE8, Firefox, Chrome and Opera all use DOM order, not character stream order.

Finally HTML5 offers pretty much no guarantees about the tab order between elements that have a tabindex of 0, merely stating that it should follow platform conventions.

Where do tabindex="0" HTML elements end up in the tabbing order?

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