这只能通过javascript设置size =占位符长度来完成：

This can be done only via javascript by setting the size = placeholder length:

input.setAttribute('size',input.getAttribute('placeholder').length);

这里是所有输入的代码：
http://jsfiddle.net/KU5kN/

Here is a code that dose that for all the inputs: http://jsfiddle.net/KU5kN/