test

foo

sample1.html

<!DOCTYPE HTML>
<html lang="ja">
  <head>
    <meta charset="UTF-8">
    <title>test</title>
    <style>
    #foo, #bar{ color: #f00; }
    </style>
  </head>
  <body>
    <div id="foo">foo</div>
    <div><iframe src="iframe1.html"></iframe></div>
  </body>
</html>

iframe1.html

<!DOCTYPE HTML>
<html lang="ja">
  <head>
    <meta charset="UTF-8">
    <title>iframe</title>
  </head>
  <body>
    <div id="bar">bar</div>
  </body>
</html>