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

From: Simon Bates <simon.bates@utoronto.ca>
Date: Fri, 14 Dec 2007 16:55:25 -0500
Message-ID: <4762FBCD.8080107@utoronto.ca>
To: wai-xtech@w3.org
CC: "fluid-work@fluidproject.org" <fluid-work@fluidproject.org>

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
Received on Friday, 14 December 2007 21:58:01 GMT

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