The CSS for the pure HTML/CSS modal I tweeted about.

pure_html_css_modal.css

details summary {
  cursor: pointer;
  outline: none !important;
  display: inline-block;
  padding: 8px 12px;
  padding-top: 10px;
  border-radius: 4px;
  overflow: hidden;
  background: #F09825;
  color: white;
  font-family: Arial;
  font-weight: bold;
  font-size: 1.2em;
  letter-spacing: 0.025em;
  text-transform: uppercase;
}

details[open] > summary::before {
  position: fixed;
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
    display: block;
    cursor: default;
    content: " ";
    z-index: 99;
    background: rgba(27,31,35,0.5);
}

details > div {
    left: 50%;
    margin: 10vh auto;
    max-height: 80vh;
    max-width: 90vw;
    position: fixed;
    top: 0;
    transform: translateX(-50%);
    width: 448px;
    z-index: 999;
    animation-name: fadein;
    animation-duration: 300ms;
    animation-timing-function: ease-in-out;
    background-color: #fff;
    border: 1px solid #d1d5da;
    border-radius: 3px;
    font-size: 2em;
    font-weight: bold;
    font-family: Arial;
    padding: 1em;
    box-shadow: 1px 1px 10px rgba(0,0,0,0.3);
    border-top: 4px solid #F09825;
}

@keyframes fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}

details summary::-webkit-details-marker {
  display:none;
}