correct summary styling

[scottaohara]

closes #891

This revised style ensures that only summary elements that are properly used will be provided the display: list-item style.

[garrettw]

Adding this to my normalize in https://github.com/garrettw/vitals

[samuelreichor]

I would suggest du completely hide the default detail marker like that:

summary::-webkit-details-marker {
 display: none;
}

summary {
 list-style: none;
}