Hi John,
I think this problem is, unfortunately, squarely an issue for the HTML specification, and also means filing bugs on browsers if they aren't already there.
Note that it isn't new: https://github.com/w3c/html/issues/292 describes the problem, and shows that people have known about it for quite a while.
It certainly seems to me that Firefox has the right behaviour, and everyone else doesn't.
For that reason, I will start a new thread about this on public-html - the public email list for the HTML specification.
I also encourage the APA group to look carefully at the issue mentioned above.
19.04.2017, 23:16, "John Foliot" <john.foliot@deque.com>:
Greetings CSS and Web Platform Working Groups,
Recently, one of our Subject Matter Experts at Deque created a test page that unearthed a... question. After a careful review of the specs (and a quick double-check with our buddy Glazou), and he recommended to bring this to you for some input, as there may be a gap here or unintended behavior - an undefined collision in Daniel's words.
First, the test page can be found at http://a11yideas.com/testcode/scrollareas.html
There are two scrollable <div> elements with no focusable elements contained within.  The first <div> has a tabindex="0", the second <div> does not. 
Both <div>s have the following CSS declaration:
.scroll {
 background-color: #ddd;
 height: 140px;
 width: 200px;
 overflow: auto;
 padding: 6px;
 margin: 10px;

The current spec for 'overflow' states:
The behavior of the 'auto' value is user agent-dependent, but should cause a scrolling mechanism to be provided for overflowing boxes.
Our testing shows that, indeed, in every mainstream browser we've tested, because the content is longer than the declared height of the <div> that the browsers are adding the scroll bars.
HOWEVER, with the exception of Firefox, if a content author does not explicitly also set the tabindex="0" to the <div> (even though the browser has added scroll bars) that keyboard-only users cannot tab "into" the scrollable <div>, nor can they scroll that <div>'s content via the keyboard only.
As you can imagine, this is a significant accessibility concern. Yet, when I read the spec, it doesn't seem to address this scenario, at least not specifically.
At the heart of the question is: if a browser adds a function that requires user-input/interaction, should the browser also be responsible for adding that 'modified' element to the tab order when appropriate (like in this example)?
Logic would suggest to me that the current Firefox behavior is the correct behavior, since the user-agent (browser) is adding the scroll bars 'when required' (per "auto"), and when the browser adds those scroll bars, that the scroll bars should also be automatically added to the tab-order. But the spec is silent on that, and to date the only browser that appears to be doing this is FF.
Alan and Rossen, I know that the CSS WG is in Tokyo for a F2F this week, and perhaps you might have time to discuss this there? 
At this point, I am unsure where I should file a bug (if, indeed, a bug is the right response), but today we have inconsistent behavior in the browsers, and exclusive of Firefox, I believe there is an unintended but serious accessibility miss in the current CSS Spec. Perhaps what the spec should say is something like:
The behavior of the 'auto' value is user agent-dependent, but should cause a tab-focusable scrolling mechanism to be provided for overflowing boxes.
However, I'll leave that to either CSS WG or Web Plat WG to work out: ideally I'd like to just know what the *intent* was of either WG (and I'm guessing that CSS WG is the better group to answer the question). 
Currently, in the absence of a definitive declaration, here at Deque we will be failing any scrollable <div> without tabindex="0", as without that declaration the scrollable <div> is unusable to keyboard-only users, but we'd love to have a specific spec to point to (and point our clients to as well).
John Foliot
Principal Accessibility Strategist
Deque Systems Inc.
Advancing the mission of digital accessibility and inclusion
Charles McCathie Nevile - standards - Yandex
chaals@yandex-team.ru - - - Find more at http://yandex.com