<header>
  <div id="header-left">
    <button class="header-button">Hello
    </button>
  </div>
</header>
.header-button{
  background-color: green;
}
header div#header-left .header-button{
  background-color: red;
}
button.header-button{
  background-color: orange;
}

External CSS

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External JavaScript

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