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    var large = document.getElementById('large');
    var box1  = document.getElementById('box1');
    var box2  = document.getElementById('box2');
  
    large.style.backgroundColor = 'black';
    box1.innerHTML = '文字色は赤、文字サイズは24p';
    box1.style.color = 'red';
    box1.style.fontSize = '24px';
    box2.innerHTML = '文字色は白、文字サイズはデフォルト';
    box2.style.color = 'white'; 

  <div id="large">
    <div id="box1">1行目(赤文字)</div>
    <div id="box2">2行目(白文字)</div>
  </div>