<button type="button" class="open-modal">Open modal</button>

<div class="modal-outer is-hidden">
  <div class="modal-inner">
    <button type="button" class="modal-close">&times;</button>
    
    <h1>Modal content</h1>
    <p>You can click anywhere outside the modal to close it.<br>Alternatively, there's the close button in the upper right corner.</p>
  </div>
</div>

body {
  margin: unset;
  font: normal 16px/1.5 Gelasio, serif;
  text-align: center;
}

.open-modal {
  font-family: inherit;
  font-size: 23px;
  background-color: tomato;
  border: none;
  font-weight: bold;
  padding: .5rem 4rem;
  border-radius: .2rem;
  margin-top: 3rem;
  cursor: pointer;
}

.open-modal:hover {
  background-color: black;
  color: white;
}

.modal-outer {
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  background-color: rgba(200, 200, 200, .4);
  display: flex;
  align-items: center;
  justify-content: center;
}

.modal-outer.is-hidden {
  display: none;
}

.modal-inner {
  background-color: white;
  position: relative;
  box-shadow: 0 10px 20px -10px rgba(0, 0, 0, .1);
  width: 100%;
  max-width: 70vw;
  max-height: 50vh;
  border-radius: .5rem;
  padding: 2rem;
  box-sizing: border-box;
  text-align: center;
}

.modal-close {
  position: absolute;
  top: 0;
  right: 0;
  font-size: 2rem;
  background-color: transparent;
  border: none;
  margin: unset;
  width: 50px;
  height: 50px;
  cursor: pointer;
}

.modal-close:hover,
.modal-close:focus {
  color: tomato;
}

(function() {
  "use strict";
  
  var modal = document.querySelector(".modal-outer");
  var open = document.querySelector(".open-modal");
  var close = modal.querySelector(".modal-close");
  
  function handleModalOpen() {
    modal.classList.remove("is-hidden");
  }
  
  function handleModalClose() {
    modal.classList.add("is-hidden");
  }
  
  function handleModalClick(evt) {
    if (!evt.target.closest(".modal-inner")) {
      handleModalClose();
    }
  }
  
  function handleKeyDown(evt) {
    if (evt.key == "Escape" && !modal.classList.contains("is-hidden")) {
      handleModalClose();
    }
  }
  
  open.addEventListener("click", handleModalOpen);
  close.addEventListener("click", handleModalClose);
  modal.addEventListener("click", handleModalClick);
  window.addEventListener("keydown", handleKeyDown);
})();