选择控件专属属性，默认选中。
<form>
<!-- radio -->
one <input type="radio" checked name="num" value="1"> <!-- 默认选中 -->
two <input type="radio" name="num" value="2">
<!-- checkbox -->
one <input type="checkbox" checked name="num" value="1"> <!-- 默认选中 -->
two <input type="checkbox" name="num" value="2">
</form>控件效果