div {
  transition: background-color 0.5s;
  background-color: transparent;
  
  display: inline-block;
  height: 2em;
  width: 2em;
  outline: 1px solid #ccc;
  margin: 0.5em;
  
  @starting-style {
    background-color: yellow;
  }
}


/* Non-demo styles below */
@layer base {
  @layer reset {
    * {
      box-sizing: border-box;
    }
    body {
      margin: 0;
      padding: 0;
    }
    html, body {
      min-height: 100%;
    }
  }
  @layer layout {
    html {
      max-width: 90ch;
      padding: 3rem 2rem;
      margin: auto;
/*      line-height: 1.5;
      font-size: 1.25rem; */
    }

    a,
    a:visited {
      color: blue;
    }
    
    h2 {
      margin-top: 2em;
    }
    
    footer {
      margin-top: 4em;
      text-align: center;
      font-style: italic;
    }

  }
  
  @layer code {
    pre {
      border: 1px solid #dedede;
      padding: 1em;
      background: #f7f7f7;
      font-family: "Courier 10 Pitch", Courier, monospace;
      overflow-x: auto;
      border-left: 0.4em solid cornflowerblue;
      tab-size: 4;
    }
    
    code:not(pre code), output:not(code:has(output) output) {
      background: #f7f7f7;
      border: 1px solid rgb(0 0 0 / 0.2);
      padding: 0.1rem 0.3rem;
      margin: 0.1rem 0;
      border-radius: 0.2rem;
      display: inline-block;
    }
  }
}

document.querySelector('button').addEventListener('click', e => {
  const $newDiv = document.createElement('div');
  document.querySelector('main').appendChild($newDiv);
});