(X)HTML 中未定义的行为？
(X)HTML 中是否存在未定义行为之类的事情？
在使用
但我注意到也可以使用 标签。完整示例：
<button>
normal text
<b>bold text</b>
<a href="http://www.example.com/">linked text</a>
</button>
在 Firefox 上呈现如下：
在 Google Chrome 中：
现在，在 Firefox 上，链接目标不可可点击，只能点击按钮...但是，在 Chrome 上，链接可点击，并将重定向到 IANA RFC2606 页面。
这是未定义的行为吗？ (X)HTML 中是否还有更多可以被描述为未定义行为的情况？
Is there such a thing as undefined behaviour in (X)HTML?
I have wondered this after playing around with the <button> tag, which allows HTML to be rendered as button. Nothing new so far...
But I noticed that one can also use the <a> tag. Complete example:
<button>
normal text
<b>bold text</b>
<a href="http://www.example.com/">linked text</a>
</button>
This is rendered as following on Firefox:
And in Google Chrome:
Now, On firefox, the link target is NOT clickable, only the button... However, on Chrome, the link is clickable, and will redirect to the IANA RFC2606 Page.
Is this undefined behaviour? Are there more cases in (X)HTML that could be described as undefined behaviour?
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评论（4）
它比仅检查 Yi Jiang 给出的 DTD 更复杂一点mu 太短。
确实，XHTML 1.0 DTD 明确禁止
元素作为问题中给出的XHTML 1.0也
符合 Strict DTD。但 Firefox 和 Chrome 之间的行为差异与问题中的按钮片段相同。
现在，我们知道 DTD 在描述后代关系的限制方面存在问题，因此上述示例符合 DTD 也就不足为奇了。
然而。 XHTML 1.0 规范的附录 B 除了以下之外还规范地描述了后代限制DTD。它说：
请注意，它不包含
元素的排除。因此，XHTML 1.0 似乎并不禁止元素成为这种遗漏几乎肯定是一个错误。
元素应该位于附录 B 中禁止作为按钮后代的元素列表中。HTML5（包括 XHTML5）在这个问题上更加彻底。它说：
其中 交互式内容 定义为
因此在 (X)HTML5 中
元素禁止成为It's a little more complex than just inspecting the DTD as given by Yi Jiang and mu is too short.
It's true that the XHTML 1.0 DTDs explicitly forbid
<a>elements as children of<button>elements as given in your question. However it does not forbid<a>elements as descendants of<button>elements.So
is XHTML 1.0 Strict DTD conforming. But it has the same behavioural difference between Firefox and Chrome as the button fragment in the question.
Now, it is known that DTDs have problems describing limitations on descendant relationships, so it's maybe not surprising that the above sample is DTD conforming.
However. Appendix B of the XHTML 1.0 spec normatively describes descendant limitations in addition to the DTD. It says:
Note that it does not contain an exclusion for the
<a>element. So it seems that XHTML 1.0 does not prohibit the<a>element from being non-child descendant of<button>and the behaviour in this case is indeed undefined.This omission is almost certainly a mistake. The
<a>element should have been in the list of elements prohibited as descendants of button in Appendix B.HTML5 (including XHTML5) is much more thorough on the matter. It says:
where interactive content is defined as
So in (X)HTML5 the
<a>element is prohibited from being a descendant of the<button>element.HTML 4 规范 声明了
如果我对 DTD 的阅读是正确的（而且我对此并不完全熟悉），
元素被明确禁止嵌套在button中，因此您看到的是无效的 HTML，因此这是未定义的行为。The HTML 4 specifications declares the
<button>as such:Which, if my reading of the DTD is correct (and I'm not exactly familiar with this),
<a>elements are explicitly forbidden from being nested inbuttons, so what you're looking at there is invalid HTML, and therefore it is undefined behavior.XHTML 说的是关于
因此
也明确从 XHTML 中排除。XHTML says this about
<button>:So
<a>is explicitly excluded from XHTML as well. The allowable elements inside<button>appear to be pretty much the same in XHTML-1.0 as in HTML-4.0.添加到 Alohci 的好答案，但更具体地回答这个问题：如果您的 (X)HTML 无效，则行为始终根据定义，未定义。在这种情况下，浏览器可以随意解释标记（或者任何可能的情况），甚至可以拒绝它（真正的浏览器不会这样做）。
这正是标签汤引入的问题，也是 XML 严格解析规则和 HTML5 规范发展到超过 500 个页面的起源。
To add to Alohci's good answer, but more specifically to answer the question: If your (X)HTML is invalid, behaviour is always, per definitionem, undefined. In this case, browsers are free to interpret the markup as they like (or whatever chance will make out of it), or even to reject it (what no real browser does).
This is exactly the problem that the tag soup introduced and that was the origin of XML's strict parsing rules and the HTML5 spec growing to >500 pages.