W3C验证器(Wikipedia)不喜欢非void元素上的自关闭标记(以“/>”结尾的标签)。(空元素是那些可能永远不包含任何内容的元素。)它们在HTML5中仍然有效吗?
一些公认的void元素的例子:
<br />
<img src="" />
<input type="text" name="username" />
一些被拒绝的非空元素的例子:
<div id="myDiv" />
<span id="mySpan" />
<textarea id="someTextMessage" />
注意: W3C验证器实际上接受无效自关闭标记:作者最初有一个问题，因为一个简单的拼写错误(\>而不是/>);然而，一般来说，自关闭标签在HTML5中并不是100%有效的，并且答案详细阐述了各种HTML风格的自关闭标签的问题。
当前回答
(Theoretically) in HTML 4, <foo / (yes, with no > at all) means <foo> (which leads to <br /> meaning <br>> (i.e. <br>>) and <title/hello/ meaning <title>hello</title>). I use the term "theoretically" because this is an SGML rule that browsers did a very poor job of supporting. There was so little support (I only ever saw it work in emacs-w3m) that the spec advises authors to avoid the syntax. In XHTML, <foo /> means <foo></foo>. This is an XML rule that applies to all XML documents. That said, XHTML is often served as text/html which (historically at least) gets processed by browsers using a different parser than documents served as application/xhtml+xml. The W3C provides compatibility guidelines to follow for XHTML as text/html. (Essentially: Only use self-closing tag syntax when the element is defined as EMPTY (and the end tag was forbidden in the HTML spec)). In HTML5, the meaning of <foo /> depends on the type of element: On HTML elements that are designated as void elements (essentially "An element that existed before HTML5 and which was forbidden to have any content"), end tags are simply forbidden. The slash at the end of the start tag is allowed, but has no meaning. It is just syntactic sugar for people (and syntax highlighters) that are addicted to XML. On other HTML elements, the slash is an error, but error recovery will cause browsers to ignore it and treat the tag as a regular start tag. This will usually end up with a missing end tag causing subsequent elements to be children instead of siblings. Foreign elements (imported from XML applications such as SVG) treat it as self-closing syntax.
其他回答
(非void)自关闭标签在HTML5中有效吗?
当然，它们是有效的，只是稍加修改。
以自关闭标签<br>为例。
即使您编写了<br/>或<br/>，它们最终也会在浏览器中转换为<br>。
在以/>或/>结尾的自关闭标记中，/(斜杠)和空白将被忽略。
举个例子，让我们看看它在浏览器中是什么样子。
<p>这段有<br><br>和<br/><br/>然后<br/><br/> .</p>
上面的代码在浏览器中看起来如下图所示。
您可以看到所有转换为<br>。所以关闭自关闭标签是你的选择，但它们是完全有效的。
HTML5的基本行为就好像末尾的斜杠不存在一样。在HTML5语法中不存在自关闭标记。
Self-closing tags on non-void elements like <p/>, <div/> will not work at all. The trailing slash will be ignored, and these will be treated as opening tags. This is likely to lead to nesting problems. This is true regardless of whether there is whitespace in front of the slash: <p /> and <div /> also won't work for the same reason. Self-closing tags on void elements like <br/> or <img src="" alt=""/> will work, but only because the trailing slash is ignored, and in this case that happens to result in the correct behaviour.
结果是，任何在旧的“XHTML 1.0充当文本/html”中工作的东西都将继续像以前那样工作:在非void标记上的尾斜杠也不被接受，而在void元素上的尾斜杠则可以。
另外需要注意的是:可以将HTML5文档表示为XML，这有时被称为“XHTML 5.0”。在这种情况下，应用XML规则并始终处理自关闭标记。它总是需要使用XML mime类型提供服务。
然而，在此声明一下，这是无效的:
<address class="vcard">
<svg viewBox="0 0 800 400">
<rect width="800" height="400" fill="#000">
</svg>
</address>
这里的斜杠将使它再次有效:
<rect width="800" height="400" fill="#000"/>
自关闭的div将不生效。这是因为div是一个普通元素，而不是一个void元素。
根据HTML5规范，不能有任何内容的标签(称为void元素)可以自关闭*。这包括以下标签:
area, base, br, col, embed, hr, img, input,
link, meta, param, source, track, wbr
然而，上述标签上的“/”是完全可选的，因此<img/>与<img>没有区别，但<img></img>是无效的。
*注意:外部元素也可以自关闭，但我不认为这在这个答案的范围内。
实际上，在HTML中使用自关闭标记应该像您期望的那样工作。但是如果您关心如何编写有效的HTML5，那么您应该了解在两种不同的语法形式中如何使用这些标记。HTML5同时定义了HTML语法和XHTML语法，两者相似但不完全相同。使用哪一种取决于web服务器发送的媒体类型。
More than likely, your pages are being served as text/html, which follows the more lenient HTML syntax. In these cases, HTML5 allows certain start tags to have an optional / before it's terminating >. In these cases, the / is optional and ignored, so <hr> and <hr /> are identical. The HTML spec calls these "void elements", and gives a list of valid ones. Strictly speaking, the optional / is only valid within the start tags of these void elements; for example, <br /> and <hr /> are valid HTML5, but <p /> is not.
The HTML5 spec makes a clear distinction between what is correct for HTML authors and for web browser developers, with the second group being required to accept all kinds of invalid "legacy" syntax. In this case, it means that HTML5-compliant browsers will accept illegal self-closed tags, like <p />, and render them as you probably expect. But for an author, that page would not be valid HTML5. (More importantly, the DOM tree you get from using this kind of illegal syntax can be seriously screwed up; self-closed <span /> tags, for example, tend to mess things up a lot).
(在服务器知道如何以XML MIME类型发送XHTML文件的不寻常情况下，页面需要符合XHTML DTD和XML语法。这意味着自关闭标记对于那些这样定义的元素是必需的。)
