[css-font-loading-3] Match FontFace default descriptor values to @font-face defaults

[danburzo]

With the default values for @font-face descriptors (font-weight, font-width, and font-style) having been changed from normal to auto, should the FontFace interface reflect these new defaults as well?

At the moment, the situation across browser engines is that not only is the default still normal, but the FontFace() constructor rejects a { width: 'auto' } descriptor.