This adds a pseudo-class `:-moz-modal-dialog` to be able to track whether the
dialog is in the top layer in CSS.
This implements default styling for modal dialogs following the HTML standard.
Slightly off-extreme color scheme chosen for unstyled <dialog> elements.
Since these are just interpreted comments, there's 0 impact on actual code.
This removes all lines that match /* vim: set(.*)tw=80: */ with S&R -- there are
a few others scattered around which will be removed manually in a second part.
