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                <!DOCTYPE html>




  .container {

  position: relative;


  .black-box {

  position: relative;

  border: 2px solid black;

  height: 100px;

  margin: 30px;


  .gray-box {

  position: absolute;

  background: lightgray;

  height: 60px;

  width: 70%;

  left: 50px;

  top: 50px;


  .green-box {

  position: absolute;

  background: lightgreen;

  width: 35%;

  left: 270px;

  top: -15px;

  height: 100px;





  <p>Nếu hai phần tử được định vị chồng lên nhau mà không chỉ định z-index, phần tử được xác định cuối cùng trong mã HTML sẽ được hiển thị ở trên cùng:</p>

  <div class="container">

  <div class="black-box">Hộp đen</div>

  <div class="gray-box">Hộp sám</div>

  <div class="green-box">Hộp xanh</div>