This inline element is mostly a mystery - there does not seem to be any
official reference to this element and I am not positive of its
intended purpose...perhaps it is used to indicate spelling errors in
the Netscape Mail or Composer editor. I only found reference to it by
looking through the source code available on the Mozilla site.
Content is rendered with a broken maroon underline beneath it for
default font sizes and below, and a solid maroon underline for font
sizes larger than the default.