<div id="num1">num1
  <div id="num2">num2</div>
  <div id="num3">num3</div>
  <div id="num4">num4</div>
</div>

#num1 {
  color: green;
}

#num1 #num2{
  color: red;
}

#num3 {
  color: blue;
}

External CSS

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External JavaScript

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