Why are nested anchor tags illegal?

问题:

I learned that nesting anchor tags is not standards compliant HTML.

From W3:

Links and anchors defined by the A element must not be nested; an A element must not contain any other A elements.

Since the DTD defines the LINK element to be empty, LINK elements may not be nested either.

It would seem like alternatives such as those suggested in the selected answer in this question would have more of a chance of creating unexpected behavior than simply nesting the anchors would!

It also seems like overkill to make onclick event handlers just to redirect the page in JS. Not to mention using a script solution would cause problems for users browsing with scripts disabled.

EDIT

What is interesting was that I was working on a fiddle to demonstrate and I had overlooked that chrome was actually restructuring the DOM as such:

<div id="container">

    <a href="http://yahoo.com"></a>
    <div class="parent">
        <a href="http://yahoo.com">Parent Element</a>
        <a href="http://google.com">
            <div class="child">Child Element</div>
        </a>
        <a href="http://bing.com">
            <div class="child">Other Child</div>
        </a>
    </div>

</div>

I overlooked this because I saw the hover working and had my mouse on the text. Knowing this now doesn't necessarily change my question, but it sure does illustrate that it doesn't even work the way I thought.

回答1:

Keep in mind that an anchor isn't just a link, it's also something to which one can link. (Though the former use is far more common than the latter.) Quoting W3C (old, but relevant):

An anchor is a piece of text which marks the beginning and/or the end of a hypertext link.

To that end, don't think of an anchor as a link. Think of it as a point in the document which connects to (and/or from) another point (in the same document or another document, it makes no difference). Two points in some non-physical space which are connected by a non-physical thread.

Given that, anchors shouldn't contain a lot of content. If it contains a lot of content, it ceases to become a "point" and starts to become an "area." For example, imagine an anchor which when rendered takes up more space than the browser can display at once. If something links to that anchor, where should it go? The beginning? Middle? End? (Intuitively you might think the beginning, but you get the idea.)

Furthermore, anchors definitely shouldn't contain other anchors. The non-physical connections between the non-physical points can become ambiguous. Does the child anchor connect to the other point, or does the parent anchor connect to the other point? This probably doesn't result in being a big deal in most cases, since the vast majority of anchors today are one-way links from the anchor to another document. But the purpose of the anchor is more than just a one-way link to another document, so the definition continues to embody that purpose.

回答2:

The spec for <a> has a content model of:

Transparent, but there must be no interactive content descendant.

So the spec is actually more complicated than you say. This also applies to <button> -- essentially you can't have working links inside of links or buttons inside of buttons.

Unfortunately I don't have a strong answer to your question why -- I can only speculate. One clear reason is the ambiguity that it causes (e.g. which anchor should be followed when the inner one is clicked?)

This creates not only functional ambiguity, but also semantic ambiguity. The content of the <a> is a label for its hyperlink. So does this mean that the inner hyperlink is part of the label for the content of the outer link?

回答3:

Anchor nesting is not recommended by standards (although all browsers I tested it on seem to lead to the innermost click target) because of ambiguity.

It is not logic to say that a big part should lead to a certain page and in this part a small section should lead somewhere else. I still have to see any use case for this kind of behaviour. With the evolution of available scripting languages, this kind of iteration or heritage can work (javascript click events on divs for exemple), but never will a code want to direct to a certain URL and then to another (parent anchors' href).

Edit: Looking at the question you referenced in yours, stacking 2 anchors that point to the exact same href is completely useless. Just remove the nested one and everything is gonna be good. If it is because you did a CSS rule like a a{color:red;} use a span! Or even a div with display:inline but please, pretty please, an anchor should go somewhere, should not be used just for styling purpose or something.