JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box0" class="mybox">
    <span class="label">box</span>
    <div class="content">
        <div id="box1" class="mybox">
            <span class="label">innerbox</span>
            <div id="box2" class="mybox">
                <span class="label">innerinnerbox</span>
            </div>
        </div>
    </div>
</div>

<div>Output:</div>
<div id="output"></div>

JavaScript

(function( $ ) {

  $.fn.closestDown = function(selector) {

    var selection = [];

    var query = function () {
      this.children().each(function() {
        var $this = $(this);
        if ($this.is(selector)) {
          selection.push(this);
        }
        else {
          query.call($this);
        }
      });
    };
    query.call(this);

    return this.pushStack(selection, "closestDown", selector);
  };

})(jQuery);

$(function () {

  var selection = $("#box0").closestDown(".mybox");

  selection.each(function() {
    $("#output").append($(".label", this).html() + "<br />");
  });

});