html input box checked property

input --checked:

As long as the check box has checked attribute, regardless of the attribute value of true or false, or null or arbitrary value, the check box will be checked. Avoid: checked property value without the quotes

<input type="checkbox" checked>1
<input type="checkbox" checked="">2
<input type="checkbox" checked="true">3
<input type="checkbox" checked="false">4
<!--4个复选框都会被勾选 -->