This might actually make more sense in vjs.Component.prototype.dimension, and actually set auto as the value, since that's valid CSS.
https://github.com/zencoder/video-js/blob/master/src/js/component.js#L615

    if ((''+num).indexOf('%') !== -1 || (''+num).indexOf('px') !== -1 || num === 'auto') {

(add `|| num === 'auto'`)

Or would it? That might override values in stylesheets. So I guess:
    if (num === 'auto') {
      this.el_.style[widthOrHeight] = '';
    }