The dialog element is now on the backlog with a priority of medium. You can track status changes at https://dev.modern.ie/platform/status/dialogelementformodals/Mark Reed commented
This has the potential to be a good replacement for the showModalDialog() command. We have an enterprise application written in asp.net that relies heavily on this command for popup windows. The HTML 5 dialog command could give us equivalent functionality... much like an iframe.