The html code can be used in xul files. If it is needed html namespace must be defined precisely as shown in the examples with xmlns:html attribute.

Notice the closing tag in <hr/>

<?xml-stylesheet href="css1.css" type="text/css" title="css1"?>
<window align="vertical"
                xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
                xmlns:html="http://www.w3.org/TR/REC-html40">

               <html:h2>Formulas</html:h2>
                <html:hr/>
               <html:ul>
                    <html:li>H<html:sub>2</html:sub>O</html:li>
                    <html:li>Na<html:sup>+</html:sup></html:li>
               </html:ul>
</window>

xul2.xul

Notice the closing tag in <hr/>

css1.css

CSS

window {background-color: #EEFFEE}
hr {background-color:black}