The third rule will match the EM in the following fragment.

第三个规则将匹配如下语句中的em元素。

We address this case by supplementing the previous rules with a rule that sets the text color to blue whenever an em occurs anywhere within an h1.

我们这样来改进这一情况：我们对前述的规则补充一个规则，这一规则将出现在H1之内任何位置的EM元素的文本颜色设置为蓝色。