'appearance' is a shorthand for 'appearance', 'color', 'font',
  'cursor'. It sets the first to the specified value and the last
  three properties to their appropriate system value; 'none' resets
  the last three to 'inherit' and 'appearance' to 'none'. 'appearance'
  does not affect the specified or computed values of any other
  properties.

  If 'appearance' is not 'none', the UA must render the element as if
  it was the specified widget from the platform. The UA should use the
  computed values of the 'background-*', 'border-*', 'padding-*',
  'outline-*', and 'text-decoration' properties when they do not have
  their initial values and the computed values of 'color', 'font', and
  'cursor' (whatever their values) to influence the rendering where
  possible. Any values from those properties that cannot be used to
  influence the rendering must not affect the rendering at all.

    For example, the UA should not draw a second border around a
    widget that already has a border.

  If 'background-color' or 'background-image' have non-initial values
  and the UA is using their values for influencing the rendering of
  the widget, then the UA must ensure that the 'color' property is
  also used to influence the rendering.

  Other properties must not affect the handling of 'appearance' and
  must instead be applied according to normal CSS rules. In particular
  'margin', 'display', 'float', 'height', 'width', and 'line-height',
  are not ignored and affect the element as normal.

  This specification does not define the term "platform".

    For example, it could be the native graphical rendering engine of
    the operating system, or it could be a a user-agent-specific skin.
    In addition, which of several toolkits to use look could also be
    decided on an element-by-element basis depending on the values of
    the 'background-color', 'background-image', 'border-*-style', and
    'outline-style' properties, so that the author styles could be
    honored while still honoring 'appearance' even though the UA is
    unable to influence the rendering of OS native widgets.

  NOTE: The exact list of properties to be set, to use for influence,
  and to not have affected by 'appearance' given in the lists above
  may be adjusted based on UA implementor feedback.