<https://pixelcoblog.com/descarga-gratis-referencia-rapida-de-css2-y-css3-en-pdf/>
<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>
<https://pixelcoblog.com/descarga-gratis-referencia-rapida-de-css2-y-css3-en-pdf/>