<div class="open">
  <a href="#modal">Open Modal</a>
<div>
<div id="modal">
  <div class="modal__window">
      <a class="modal__close" href="#">X</a>
      <h2>Please to meet you!</h2>
      <p>Hello there, I am a nice Modal Window.</p> 
  </div>
</div>

#modal {
    position: fixed;
    top: 0;
    left: 0;
    height: 100%;
    width: 100%;
    background-color: rgba(0,0,0,.5);
    display: flex;
    justify-content: center;
    align-items: center;
}

.modal__window {
  position: relative;
  background-color: white;
  padding: 4em 2em;
}

.modal__close {
  position: absolute;
  top: -30px;
  right: 0;
}