Add missing file

Some utility functions have been moved to `src/headless/utils/html.js`

Updates #3243

src/headless/utils/html.js

@@ -0,0 +1,64 @@
+import { Strophe } from 'strophe.js';
+
+/**
+ * @param { any } el
+ * @returns { boolean }
+ */
+export function isElement (el) {
+    return el instanceof Element || el instanceof HTMLDocument;
+}
+
+/**
+ * @param { Element | typeof Strophe.Builder } stanza
+ * @param { string } name
+ * @returns { boolean }
+ */
+export function isTagEqual (stanza, name) {
+    if (stanza instanceof Strophe.Builder) {
+        return isTagEqual(stanza.tree(), name);
+    } else if (!(stanza instanceof Element)) {
+        throw Error(
+            "isTagEqual called with value which isn't "+
+            "an element or Strophe.Builder instance");
+    } else {
+        return Strophe.isTagEqual(stanza, name);
+    }
+}
+
+/**
+ * @param {HTMLElement} el
+ * @param {boolean} include_margin
+ */
+export function getOuterWidth (el, include_margin=false) {
+    let width = el.offsetWidth;
+    if (!include_margin) {
+        return width;
+    }
+    const style = window.getComputedStyle(el);
+    width += parseInt(style.marginLeft ? style.marginLeft : '0', 10) +
+             parseInt(style.marginRight ? style.marginRight : '0', 10);
+    return width;
+}
+
+/**
+ * Converts an HTML string into a DOM element.
+ * Expects that the HTML string has only one top-level element,
+ * i.e. not multiple ones.
+ * @method u#stringToElement
+ * @param {string} s - The HTML string
+ */
+export function stringToElement (s) {
+    var div = document.createElement('div');
+    div.innerHTML = s;
+    return div.firstElementChild;
+}
+
+/**
+ * Returns a list of children of the DOM element that match the selector.
+ * @method u#queryChildren
+ * @param {HTMLElement} el - the DOM element
+ * @param {string} selector - the selector they should be matched against
+ */
+export function queryChildren (el, selector) {
+    return Array.from(el.childNodes).filter(el => (el instanceof Element) && el.matches(selector));
+}