main.css

body {
  width: 100%;
  font-family: 'Arial', sans-serif;
  background-color: #FFFEE5;
  margin: 0;
  display: flex;
  /* good layout */
  display: grid;
  /* good layout */
  display: inline-flex;
  /* good layout */
}
div {
/*  border: 4px solid black;*/
/*  border-radius: 4px;*/
}
h3 {
  text-align: center;
}
#parent {
  position: inherit;
  display: inline-block;
  border: 3px solid blue;
  width: 25%;
  top: 0px;
  max-height: 80%;
  /*border: 3px solid blue;* /
  white-space: nowrap;
  overflow-y: hidden;
  overflow-x: scroll;
  /*padding: 1rem;*/
}
#playground {
  display: block;
  border: 3px solid orange;
  width: 80%;
  /*border: 3px solid blue;* /
  white-space: nowrap;
  overflow-y: hidden;
  overflow-x: scroll;
  /*padding: 1rem;*/
}
#selectors {
  display: inline-block;
  border: 3px solid purple;
  padding-left: .25em;
  width: 60%;
  overflow: scroll;
  max-height: 80%;
  /*border: 3px solid blue;* /
  white-space: nowrap;
  overflow-y: hidden;
  overflow-x: scroll;
  /*padding: 1rem;*/
}