mark 元素用于标注一段文本，这段文本会突出显示。在需要引用其他人的内容，而你想引起别人注意时，就可以使用 mark元素.
也可以使用 mark标注重要的内容或关键字??就像搜索引擎在搜索结果中显示匹配的文本那样，还可以与 del （删除的文本）和 ins（插入的文本）组合使用，以标注文档的变化。
实话实说， mark 元素其实并不十分必要。HTML5规范认为它是一个语义元素，但实际上它更多地则是被用于表现目的。默认情况下，被标注的文本会带有浅黄色背景（见图3-2），不过当然可以通过样式表规则为它应用不同的样式。
提示 mark 元素的目的并不在于样式。毕竟，突出显示文本的方式已经有很多了。因此，应该坚持只用 mark （结合你想使用的任意css格式）来传达适当的语义。这里有一条经验，那就是使用 mark 来吸引人注意那些变得很重要的文本。比如，关于写作该文本的>讨论，或者它是用户应该执行的任务。
<mark> Example In 2009, Facebook made a bold grab to own everyone's content,forever. This is the text they put in their terms of service:
You hereby grant Facebook an irrevocable, perpetual, non-exclusive, transferable, fully paid, worldwide license (with the right to sublicense) to use, copy, publish, stream, store, retain, publicly perform or display, transmit, scan, reformat, modify, edit, frame, translate, excerpt, adapt, create derivative works and distribute (through multiple tiers), any user content you post on or in connection with the Facebook Service or the promotion thereof subject only to your privacy settings or enable a user to post.Fortunately, they've since backtracked and weakened the language considerably.Here's the relevant section today:
You own all of the content and information you post on Facebook, and you can control how it is shared through your privacy and application settings. In addition: 1. For content that is covered by intellectual property rights, like photos and videos ("IP content"), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any content that you post on or in connection with Facebook. This license ends when you delete your content or your account unless your content has been shared with others, and they have not deleted it.