Use a default font size when it can't be computed directly (e.g., the element is not in an active DOM). (mathjax/MathJax#3458)

Author: dpvc

ts/adaptors/HTMLAdaptor.ts

@@ -199,6 +199,13 @@ export class HTMLAdaptor<
   extends AbstractDOMAdaptor<N, T, D>
   implements MinHTMLAdaptor<N, T, D>
 {
+
+  /**
+   * The font size to use when it can't be measured (e.g., the element
+   * isn't in the DOM).
+   */
+  public static DEFAULT_FONT_SIZE = 16;
+
   /**
    * The HTML adaptor can measure DOM node sizes
    */
@@ -588,7 +595,10 @@ export class HTMLAdaptor<
    */
   public fontSize(node: N) {
     const style = this.window.getComputedStyle(node);
-    return parseFloat(style.fontSize);
+    return parseFloat(
+      style.fontSize ||
+      String((this.constructor as typeof HTMLAdaptor).DEFAULT_FONT_SIZE)
+    );
   }
 
   /**