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answer from stackflow:

simply <br> is sufficient.

the other forms are there for compatibility with xhtml; to make it possible to write the same code as xhtml, and have it also work as html. some systems that generate html may be based on xml generators, and thus not have the ability to output just a bare <br> tag; if you're using such a system, it's fine to use <br/>, it's just not necessary if you don't need to do it.

very few people actually use xhtml, however. you need to serve your content as application/xhtml+xml for it to be interpreted as xhtml, and that will not work in ie (it will also mean that any small error you make will prevent your page from being displayed, in browsers that do support xhtml). so, most of what looks like xhtml on the web is actually being served, and interpreted, as html. see serving xhtml as text/html considered harmful for some more information.

意思就是說:html中的規定是<br>空標簽，沒有結束符，但是在xhtml中，規定是<br/>開始標簽后面緊跟結束標簽。

在 xhtml 中，<br> 標簽必須被正確地關閉，比如這樣：<br />。<br> 標簽是空標簽（意味著它沒有結束標簽，因此這是錯誤的：<br></br>）。在 xhtml 中，把結束標簽放在開始標簽中，也就是 <br />。