jQuery.css 会找到在页面上具有 Foo 类的所有现有元素，然后设置其内联样式 width 到 40px 。

jQuery.css will find all existing elements on the page that have the Foo class, and then set their inline style width to 40px.

换句话说，更改css规则 - 如果你动态添加一个元素与 Foo 类，它仍然会有一个宽度 20px ，因为其内联样式未设置为覆盖默认CSS规则。

In other words, this doesn't create or change a css rule -- if you dynamically add an element with the Foo class, it would still have a width of 20px, because its inline style hasn't been set to override the default CSS rule.

而应使用 addClass 和 removeClass 并控制静态CSS中的样式。

Instead, you should use addClass and removeClass and control the styles in your static CSS.