I think aria spec and html spec are inconsistent.

http://w3c.github.io/aria/html-aam/html-aam.html#summary-and-details-elements
"6.1 summary and details Elements
6.1.1 Focus and Keyboard Interaction

The summary element should be focusable by default.

The details element should not focusable by default.

Pressing the spacebar or enter key when the summary element has focus will show the details element content if the content is hidden. If the details element content is showing and the summary element has focus, pressing the spacebar or enter key will hide the details element content."

https://html.spec.whatwg.org/multipage/dom.html#interactive-content
"3.2.4.2.7 Interactive content

Interactive content is content that is specifically intended for user interaction.

    a (if the href attribute is present)audio (if the controls attribute is present)buttondetailsembediframeimg (if the usemap attribute is present)input (if the type attribute is not in the Hidden state)keygenlabelobject (if the usemap attribute is present)selecttextareavideo (if the controls attribute is present)

The tabindex attribute can also make any element into interactive content."

ARIA spec says that summary element should be focusable but HTML spec says that details element, not summary, is an interactive content.

I'm working on chromium AX as an external contributor, there is a bug report, https://bugs.chromium.org/p/chromium/issues/detail?id=389461#c6, related to event handling with details and summary elements.

Please take a look.