I don’t know if it is all right, to make a fresh post …
A little late but finally ready: test cases for CSS flex box layout and its accessibility issues.
http://sprungmarker.de/wp-content/uploads/css-a11y-group/css-a11y-flexbox.html
As expected flex box possibilities to change content order is critical to keyboard users.
All browser and screen reader use for keyboard use source code order and not visual content order generated with CSS.
This can be quite confusing and depends on use case of reordering content. At worst it can be more like a zigzag on the page.
Only current Firefox is using visual order in old flex box specification and ordinal-group.
New spec is hard to test because only chrome is supporting it. Internet Explorer 10 and some versions of Webkit nightly do too. So I have to keep an eye on development and update test cases when browser get more along with the new spec.
Also we have to consider that browser and screen reader might adapt on visual content flux with flex box. No one knows.
I will keep test case updated.
Sylvia