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HTML 

              
                <div class="content-wrapper">
  
  <p>A simple modal without any JavaScript whatsoever.</p>
  
  <p>Try it out!</p>
  
  <br />
  
  <label>
    <span class="btn">Click here to open the confirm</span>
    <input type="checkbox" />
    <div class="confirm-modal">
      <form action="#do-something" method="get">
        <p>Are you absolutely sure you know what you're doing?</p>
        <button class="btn">Yes</button>
        <button class="btn" formaction="#do-something-else">No</button>
      </form>
    </div>
  </label>
  
</div>
!

CSS 

              
                /* All elements should be border-boxes */
* {
  box-sizing: border-box;
  font-family: 'Open Sans', sans-serif;
  font-weight: 100;
  color: #8a8a8a;
}

/*
  Hide the checkbox, as it's only works as the switch
  for the confirm-modal.
*/
input[type=checkbox] {
  display: none;
}

p, button, .btn {
  font-size: 20px;
}

body {
  background-color: #fafafa;
  background: url(https://images.unsplash.com/photo-1432256851563-20155d0b7a39?crop=entropy&dpr=1.75&fit=crop&fm=jpg&h=800&ixjsv=2.1.0&ixlib=rb-0.3.5&q=57&w=1200) no-repeat center center fixed; 
  background-size: cover;
}

button {
  outline: none !important;
}

/* Wraps everyting, nothing special */
.content-wrapper {
  width: 100%;
  // max-width: 100%;
  padding: 10px;
  margin: 0 auto;
  text-align: center;
}

/*
  The sort of button-ish thingy.
  It should be wrapped by a label and
  should be followed by the checkbox.
*/
.btn {
  transition: background-color 0.2s;
  background-color: transparent;
  color: #8a8a8a;
  min-width: 3em;
  padding: 10px;
  cursor: pointer;
  border-radius: 5px;
  border: 1px solid #8a8a8a;
  user-select: none;
  display: inline-block;
  width: 300px;
  max-width: 90%;
  margin: 0 auto;
  
  &:hover {
    color: #fafafa;
    background-color: #8a8a8a;
  }
  &:active, &:focus {
    color: #fafafa;
    background-color: #a0a0a0;
  }
}

/*
  The modal background (the dark-ish overlay),
  which obviously could be a class instead of a form.
  
  For scroll to work as expected, the position is absolute,
  as position: fixed would not allow the page to scroll when the screen isn't tall enough.
*/
.confirm-modal {
  display: block;
  pointer-events: none;
  transition: opacity 0.5s;
  .btn {
    width: auto;
    max-width: auto;
    margin: auto;
  }
  
  &::before {
    /*
      The shadow/backdrop which also is clickable to close the modal.
    */
    content: "";
    display: block;
    background-color: rgba(0,0,0,0.4);
    position: absolute;
    top: 0;
    bottom: 0;
    left: 0;
    right: 0;
    min-height: 200px;
    z-index: 99;
    opacity: 1;
    pointer-events: all;
  }
  form {
    /*
      The actual modal.

      Note: to not have clicks inside the modal
      bleed through to the label (which would uncheck the checkbox),
      pointer-events are set to none on the modal.
    */
    position: absolute;
    z-index: 99;
    top: 50px;
    transition: transform 0.75s;
    max-width: 90%;
    width: 450px;
    background-color: #fafafa;
    padding: 20px;
    left: 50%;
    transform: translate(-50%, 0);
    box-shadow: 4px 4px 15px rgba(0,0,0,0.2);
    color: #080808;
    border-radius: 5px;
    pointer-events: none;
  }
}

/* Hide the modal when the checkbox isn't checked. */
input:not(:checked) + .confirm-modal {
  /* Uncomment this for a sharper switch, which might work better on older browsers. */
  /* display: none; */
  
  /*
    Might be wonky on older browsers, but allows for the fly down.
    Also does the fade in.
  */
  &, & * {
    opacity: 0; 
  }
  &::before {
    height: 0;
    width: 0;
  }
}

/* For that cute little fly down effect */
input:not(:checked) + .confirm-modal form {
  transform: translate(-50%, -100px);
}

/* Allow buttons to be clickable. */
.confirm-modal button {
  pointer-events: all;
}
!

JS 

              
                /*********************
 * Look, nothing here!
 *********************/
!
999px

Console