Created attachment 209649 [details]
HTML file highlighting the problem
It seems that similarly to bug 115861, when the children of an element with contentEditable="true" are flowed in the element itself, the contentEditable attribute is lost and they content of the element is no longer editable.
See the attached sample - the text in the black bordered rectangle should be editable, but it's not. What's interesting is that the blue highlight is still displayed when it received focus but the content is still not editable.