You’re referring to a DSL written in Kotlin for constructing HTML through the builder. The library can be found here: https://github.com/Kotlin/kotlinx.html
Here's a running example:
fun main(args: Array<String>) {
    val document = DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument()
    val html = document.create.html {
        head {
            title("Hello world")
        }
        body {
            h1("h1Class"){
                +"My header1"
            }
            p("pClass"){
                +"paragraph1"
            }
        }
    }
   intoStream(html, System.out)
}
fun intoStream(doc: Element, out: OutputStream) {
    with(TransformerFactory.newInstance().newTransformer()){
        setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no")
        setOutputProperty(OutputKeys.METHOD, "xml")
        setOutputProperty(OutputKeys.INDENT, "yes")
        setOutputProperty(OutputKeys.ENCODING, "UTF-8")
        setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "4")
        transform(DOMSource(doc),
                StreamResult(OutputStreamWriter(out, "UTF-8")))
    }
}
And finally here's the corresponding output:
<?xml version="1.0" encoding="UTF-8"?><html>
<head>
    <title>Hello world</title>
</head>
<body>
    <h1 class="h1Class">My header1</h1>
    <p class="pClass">paragraph1</p>
</body>
</html>