public string GetClassString()
        {
            var builder = new CssBuilder(ClassBase);

            if (UnknownAttributes != null &&
                UnknownAttributes.TryGetValue(
                    Attributes.Class, out object @class))
            {
                builder.Add(@class.ToString());
            }

            GetClassString(builder);
            return(builder.Build());
        }
 /// <summary>
 /// Adds a conditional nested CssBuilder to the builder with space separator.
 /// </summary>
 /// <param name="builder">CSS Class to conditionally add.</param>
 /// <param name="when">Condition in which the CSS Class is added.</param>
 /// <returns>CssBuilder</returns>
 public CssBuilder AddClass(CssBuilder builder, bool when = true) => when?AddClass(builder.Build()) : this;