<main>
  <div class="wrapper">
    <article class="flow">
      <h1>Matching specificity</h1>
      <figure class="callout">
        <p>The second CSS rule wins because it gets an extra 1 point of specificity.</p>
      </figure>
      <button class="my-button" onclick="alert('hello')">Click me</button>
    </article>
  </div>
</main>

.my-button {
  background: blue;
}

button[onclick] {
  background: grey;
}