static CursorMode Evaluate(CSSValue value)
        {
            CursorMode mode;

            if (modes.TryGetValue(value, out mode))
            {
                return(mode);
            }

            var location = value.ToUri();

            if (location != null)
            {
                return(new CustomCursorMode(new Url(location)));
            }

            return(null);
        }