panel.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;
}