问违反z-索引，并强制元素前面无论如何(或假的)

EN

var renderClonedEl = function($el, childrenToCopy) {
    var $clone = $($el[0].cloneNode(true)),
        offset = $el.offset(),
        styles = window.getComputedStyle($el[0], null).cssText,
        correction = {
            left: parseInt($el.css('margin-left')) + parseInt($el.css('border-left-width')),
            top: parseInt($el.css('margin-top')) + parseInt($el.css('border-top-width'))
        },
        $child, $cloneChild;

    childrenToCopy = childrenToCopy || []; // Use array of selector strings to copy addional styles between children elements
    $clone[0].style.cssText = styles;

    for (var i = 0; i < childrenToCopy.length; i++) {
        $child = $el.find(childrenToCopy[i]);
        $cloneChild = $clone.find(childrenToCopy[i]);

        _.each($cloneChild, function(item) {
            item.style.cssText = window.getComputedStyle($child[0], null).cssText;
        });
    };

    $clone.addClass('el-clone').appendTo('body').css({
        top: offset.top - correction.top,
        left: offset.left - correction.left
    });

    return $clone;
}

.el-clone {
    position: absolute;
    z-index: 9999;
    pointer-events: none;
}

var $newClone = renderClonedEl($('.el-to-clone'), ['.header > .title', '.header .btn']);

function Obscurer() {
  this.cont = document.createElement('div')
  this.e1 = this.cont.appendChild(this.cont.cloneNode())
  this.e1.className = 'cover'
  this.e2 = this.cont.appendChild(this.e1.cloneNode())
  this.e3 = this.cont.appendChild(this.e1.cloneNode())
  this.e4 = this.cont.appendChild(this.e1.cloneNode())
}
Obscurer.prototype.obscureSurroundingsOf = function(el) {
  if (el.parentNode === this.cont) {
    document.body.removeChild(this.cont)
    return
  }
  var b = el.getBoundingClientRect()
  this.e1.style.width = b.left + b.width + 'px'
  this.e1.style.height = b.top + 'px'
  this.e2.style.left = this.e1.style.width
  this.e2.style.height = b.top + b.height + 'px'
  this.e3.style.top = this.e2.style.height
  this.e3.style.left = b.left + 'px'
  this.e4.style.top = b.top + 'px'
  this.e4.style.width = b.left + 'px'
  document.body.appendChild(this.cont)
}

var o = new Obscurer()

document.addEventListener('click', function(ev) {
  o.obscureSurroundingsOf(ev.target)
})

.cover {
  position: absolute;
  background-color: #000;
  opacity: 0.5;
  top: 0;
  left: 0;
  bottom: 0;
  right: 0;
}

<p>foo baz</p>
<p>baz <b>gazonk</b> <i>something</i></p>
<p>anything</p>