Proposal for https://imgur.com/a/uZnjLyb

testcase-reducer.css

* {
  padding: 0;
  margin: 0;
}
html,
body {
  height: 100%;
  overflow: hidden;
  width: 100%;
}
body {
  display: grid;
  grid-template-columns: 1fr 1fr;
  grid-template-rows: fit-content(100%);
}
#controls,
#exports {
  padding: 4px;
}
textarea,
#preview {
  border-color: #000;
  border-style: solid;
  border-width: 1px 0 0 0;
}
#exports {
  display: flex;
}
#exports > button {
  margin-left: 4px;
}
.flexSpace {
  flex-grow: 1;
}
#preview {
  background: url("data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAUAQMAAAC3R49OAAAABlBMVEXv7+////9mUzfqAAAAFElEQVQIW2NksN/ISAz+f9CBGAwAxtEddZlnB4IAAAAASUVORK5CYII=");
  overflow: auto;
}
textarea {
  border-right-width: 1px;
  padding: 4px;
  resize: none;
}
iframe {
  background: #fff;
  border: 0;
}
label > input {
  margin-right: 0.5em;
  position: relative;
  top: 0.25em;
}

/* Some proposals */

#controls {
  display:flex;
  padding: 1px;
}

#reduce {
  background-color: #fc0;
  -moz-appearance: none;
  -webkit-appearance: none;
  border: none;
  border-radius: 3px;
  text-transform: uppercase;
  font-weight: bold;
  padding: 12px 13px 11px;
  text-align: center;
  display: inline-block;
  height: 100%;
}

label {
  display:flex;
  font-family: sans-serif;
  font-size: .8em;
  margin: 0 1px;
  padding: 4px;
  border: none;
  border-radius: 3px;
  background-color: #6D8C2E;
  color: white;
}

#showSameViewportSize {
  background-color: #C45A20;
  width: 25%;
  text-transform: uppercase;
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100%;
  margin-left: -4px;
  margin-top: -3px;
}

#preview {
  border: 2px solid #C45A20;
}
textarea {border: none;
	font-family: monospace;
	font-size: 16px;
	line-height: 2.5ch;
	white-space: pre-wrap;
	background-color: hsla(0, 0%, 95%, 1);
	padding: 1ch;
	background-position: 0 1ch;
	background-size: 1ch 2.5ch;
	background-image: linear-gradient(hsla(0, 0%, 99%, 1) 2px, transparent 1px), linear-gradient(90deg, hsla(0, 0%, 98%, 1) 1px, transparent 1px);
}