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<form>
    <label>
      <input class="checkbox" type="checkbox"/>
      同意する
    </label>
    <button class="button" type="submit">送信</button>
</form>

form button{
    text-decoration: line-through;
    padding: 8px 20px 8px;
    border: #333 solid 1px;
    background: #878787;
}   
form:has(.checkbox:checked) button{
    text-decoration: none;
    background: #dbdbdb;
}

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