(does-exist, "Fallback value");
  }

  #six::after {
    content: attr(does-exist, "Fallback value");
  }
</style>

<div id="one"></div>
<div id="two" does-exist="Not fallback value"></div>
<div id="three"></div>
<div id="four" does-exist="Not fallback value"></div>
<div id="five" does-exist=""></div>
<div id="six" does-exist></div>