The dialog.showModal() function defines steps for setting focus [1]. However, if the dialog does not contain any elements with the autofocus attribute, then step 9 says to abort the steps and not set focus. It seems problematic if the dialog is modal and does not have focus. How would a keyboard user interact with the dialog? How would a screen reader know that the dialog has been shown?
I would expect the control element to fall back to the first tab focusable element if no element with autofocus exists. If there is still no control element, I would expect the dialog itself to receive focus. ARIA requires the element to contain at least one tab focusable element [2]. [1] http://www.whatwg.org/specs/web-apps/current-work/multipage/commands.html#dom-dialog-showmodal [2] http://www.w3.org/WAI/PF/aria-practices/#modal_dialog