Getting the tabindex value with JavaScript on elements without a tabindex attribute

[Simon Bates]

I have been writing a function for Dojo's DHTML accessibility support 
that determines if an element is tab-navigable. The function will use 
the extended tabindex usage defined by the current ARIA and HTML5 
drafts. That is, tabindex on all elements, and tabindex="-1" for removal 
from tab order. I have discovered that there is some variation across 
browsers in the value that one gets when retrieving the tabindex value 
on an element without a tabindex attribute. This note summaries what I 
have seen.

I have looked at two elements: div and input. I am using div as an 
example of an element that did not have a tabindex attribute in HTML4 
and input as an example of one that did. I document here two mechanisms 
for retrieving the tabindex: getAttribute(), and the tabIndex property. 
Please see below for a link to my test file and a link to the full results.

Firefox 2.0.0.11 Windows XP
and Minefield nightly 3.0b3pre 2007121405 Windows XP

div with no tabindex:
	elem.getAttribute("tabindex") = null
	elem.tabIndex = -1

input with no tabindex:
	elem.getAttribute("tabindex") = null
	elem.tabIndex = 0

Internet Explorer 7.0.5730.11 Windows XP
and Internet Explorer 6 Windows XP

div with no tabindex:
	elem.getAttribute("tabindex") = 0
	elem.getAttribute("tabindex", 2) = 32768
	elem.tabIndex = 0

input with no tabindex:
	elem.getAttribute("tabindex") = 0
	elem.getAttribute("tabindex", 2) = 32768
	elem.tabIndex = 0

On IE it is not possible to determine whether a div has no tabindex or 
tabindex="0" using elem.getAttribute("tabindex") or elem.tabIndex 
because a default value of 0 is returned. Microsoft provides an 
extension to getAttribute() that can be used to determine if the 
tabindex is unset. If a second parameter of value "2" is passed to 
getAttribute(), a value of 32768 is returned whenever tabindex is not 
set. See Microsoft's documentation on getAttribute:

http://msdn2.microsoft.com/en-us/library/ms536429.aspx

I have been unable to find documentation on the value of 32768 as used 
for tabIndex but it is outside of the range of 0 to 32767 specified by 
HTML 4:

http://www.w3.org/TR/html401/interact/forms.html#adef-tabindex

Safari 3.0.4 Mac

div with no tabindex:
	elem.getAttribute("tabindex") = null
	elem.tabIndex = undefined

input with no tabindex:
	elem.getAttribute("tabindex") = null
	elem.tabIndex = 0

Test file used to gather results:

http://trac.bitstructures.com/browser/collected/trunk/html-test-cases/getting-tabindex.html?format=raw

Collected test results:

http://trac.bitstructures.com/browser/collected/trunk/html-test-cases/getting-tabindex-results.html?format=raw

Please let me know if you see any shortcomings in the test file or results.

Simon Bates
Adaptive Technology Resource Centre
University of Toronto