Como fazer um formulário com 4 checkbox onde o usuário só pode escolher 2 itens do checkbox?

[felquis]

Idealmente sem JavaScript.

      <form action="">
        <label for="a5">
          <input type="checkbox" id="a5" name="routine" value="workout regularly" />
          workout regularly
        </label>
        <label for="a4">
          <input type="checkbox" id="a4" name="routine" value="have a family" />
          have a family
        </label>
        <label for="a3">
          <input type="checkbox" id="a3" name="routine" value="contribute to open source" />
          contribute to open source
        </label>
        <label for="a2">
          <input type="checkbox" id="a2" name="routine" value="hangout with friends" />
          hangout with friends
        </label>
        <label for="a1">
          <input type="checkbox" id="a1" name="routine" value="play video games" />
          play video games
        </label>
      </form>

Obrigado desde já