Right now we use codemirror's JavaScript mode on the website but there's a JSX mode now so we should start using that instead.

var widget = !ignoreWidget && marker.replacedWith;
    if (widget) {
      if (builder.copyWidgets) widget = widget.cloneNode(true);
      builder.pre.appendChild(widget);
      if (builder.measure) {
        if (size) {
          builder.measure[builder.pos] = widget;
        } else {
          var elt = zeroWidthElement(builder.cm.display.measure);
          if (marker.type == "bookmark" && !marker.insertLeft)
            builder.measure[builder.pos] = builder.pre.appendChild(elt);
          else if (builder.measure[builder.pos])
            return;
          else
            builder.measure[builder.pos] = builder.pre.insertBefore(elt, widget);
        }
        builder.measuredSomething = true;
      }
    }
    builder.pos += size;
  }
  
  
  function insertLineContent(line, builder, styles) {