Some handy links here... need to collate

http://www-03.ibm.com/able/guidelines/software/swkbdequiv.html

I propose something like this additional text:

Putting the elements in document order should suffice. So in the case of this HTML markup:

<div id="A" tabindex="0"></div>
<div id="B" tabindex="0">
  <div id="B1" tabindex="0">1</div>
  <div id="B2" tabindex="0">2</div>
</div>
<div id="C"></div>

The order of traversal would be: A, B, B1, B2, C.

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Editor note: I'm not aware of any any cases where this would fly in the face of platform convention. In cases where this algorithm doesn't work well, DHTML authors can specify the tab order more directly (e.g. tabindex="5"), or implement special keyboard navigation depending on the desired interaction.

So is the advice for user agents to simply add the elements with tabindex="0" to the tab order in the order in which they are encountered in the source code? Or is this an example of the platform convention for HTML?

Interesting question. Let's use "platform" to describe the os/desktop (Mac, Win, Linux). I guess this is an example of convention on HTML (on all platforms AFAIK).

The user agent developer should know the convention for the technology in question. For example, in HTML, the UA will add the elements to the tab order following the source code order. Can simply remove the phrase ", and should follow platform conventions to determine the element's relative order."