style.css

body, html {
  border: 0;
  margin: 0;
  padding: 0;
}
h1 {
  background: black;
  color: white;
  font: 100 40px/1.2 'Roboto Slab', sans-serif;
  margin: 0 0 40px 0;
  padding: 40px;
  text-align: right;
}
article {
  max-width: 750px;
  margin: 0 auto;
}
h2 {
  font: 800 24px/1.2 'Open Sans', sans-serif;
}
p, ul {
  font: 18px/1.2 'Open Sans', sans-serif;
}

select {
  max-width: 100%;
}
#input, #configtext {
  display: block;
  width: 100%;
  height: 200px;
  margin: 0;
  box-sizing: border-box;
  padding: 1em;
  font-size: 1.2em;
}
  #input + p {
    text-align: right;
    margin-top: 0;
  }
  #configtext {
    font-family: monospace;
  }
#inject {
  margin: 1em 0;
}

#output { display: flex; }
#output > div {
  flex: 1;
  display: block;
  width: 750px;
  max-width: 50%;
  min-height: 200px;
  box-sizing: border-box;
  border-radius: 8px;
  padding: 1em;
  box-shadow: 0 16px 24px 2px rgb(29 17 51 / 4%), 0 6px 32px 4px rgb(9 32 77 / 12%), 0 8px 12px -5px rgb(29 17 51 / 12%);
}
#output > div:first-child {
  margin-right: 1em;
}
#output > div:nth-of-type(2) {
  font-family: monospace;
  font-size: 1.2em;
}

footer {
  background: black;
  color: white;
  margin: 1em 0 0 0;
  padding: 1ch 2ch;
  font: 15px/1.4 'Open Sans', sans-serif;
  text-align: right;
}
  footer a, footer a:visited {
    font-family: monospace;
    color: #52cbff;
  }
.warning {
  color: red;
  background: #FFFFCA;
  margin: 1.5em 0;
  padding: 0.5em 1em;
  position: relative;
  margin-top: 2em;
  padding-top: 1em;
  padding-bottom: 1em;
}
.warning::before {
  color: yellow;
  background: red;
  padding: 0.15em 0.25em;
  position: absolute;
  top: -0.8em;
  left: -0.8em;
  content: '\26A0 Warning!';
}
.invisible {
  display: none !important;
}
pre {
  margin: 0 auto 20px;
  padding: 20px 8px 20px 20px;
  font: 1.2em monospace;
  max-width: 710px;
  background: #FCFAEE;
  border: 0 solid #E0CB52;
  border-width: 0 0 0 0.5em;
  color: #555;
  white-space: pre;
  overflow: auto;
}
  pre strong { color: #000; }