解决方法
2.5.2 Boolean attributes
A number of attributes are boolean attributes. The presence of a
boolean attribute on an element represents the true value,and the
absence of the attribute represents the false value.If the attribute is present,its value must either be the empty string
or a value that is an ASCII case-insensitive match for the attribute’s
canonical name,with no leading or trailing whitespace.The values “true” and “false” are not allowed on boolean attributes. To represent a false value,the attribute has to be omitted altogether.
