Link標籤有兩個作用：1. 定義文檔與外部資源的關係；2. 是鏈接樣式表。
link標籤是用於當前文檔引用外部文檔的，其次，這個標籤的rel屬性用於設置對象和鏈接目的間的關係，說白了就是指明你鏈進來的對象是個什麼東西的，具體的值及其所表示的關係如下：
Alternate:Substitute version of the file that contains the link.
Appendix:Page that is an appendix for the set of pages.
Bookmark:Bookmark.
Chapter:Page that is a chapter for a set of pages.
Contents:Table of contents document.
Copyright:Copyright notice for the current page.
Glossary:Glossary for the current page.
Help:Help document.
Index:Index document for the current page.
Next:Next document in a sequence.
Offline:href that contains a path to the CDF file to be used for an offline favorite.
Prev:Previous document in a sequence.
Section:Page that is a section for a set of pages.
Shortcut:Icon href that contains a path to an icon file to be used for the favorite or link.
Start:First document of a set.
Stylesheet:Style sheet.
Subsection:Page that is a subsection for a set of pages.
