HTML element nesting problem

HTML element nesting problem

Element nesting

Block elements can contain inline elements or certain block elements, but inline elements cannot contain blocks Elements can only contain other inline elements.

P element nesting problem

<p>
  </p><p></p>

Nesting p and other block-level elements in the P element is parsed as follows in the browser:

You can see that there will be an empty <p></p> at the end of the element.

Through query, it was found that the p element cannot be wrapped within the block-level element .
Because the DTD we use stipulates that block-level elements cannot be placed inside <p></p>, and some browsers condone this writing method:

<p>这是一个段落的开始
</p><p>这是另一个段落的开始</p>

When a <p></p> tag has not ended, it will end itself when it encounters the next block element. In fact, the browser processes them like this:

<p>这是一个段落的开始</p>
<p>这是另一个段落的开始</p>

This also explains why there will be an extra empty <p></p> in the browser at the end.

Block-level element nesting problem

You can first divide all block elements into several levels again. We already know that is at the outermost layer, The next level will only have , , <frameset>, <noframes></noframes> </frameset>, and we already know that Visual elements will only appear in , so we put in the first level. Then, elements that cannot be freely nested are placed in the third level, and the others are placed in the second level. The so-called elements that cannot be freely nested are those that can only contain inline elements. They include: <h1>, <h2>, <h3>, <h4>, , <h6>, <caption></caption> </h6> </h4> </h3> </h2> </h1>;paragraph mark<p></p>;separator<hr>and a special element <dt></dt> (It only exists at the child level of list element <dl></dl>).

Three-level elements refers to block-level elements that can only nest inline elements. p also happens to be one of them.

Why second-level elements can be nested freely

We can think of them as some containers (or boxes), and the sizes of these containers can be changed freely. For example, we can <ul></ul> is embedded in <p></p>, or <p></p> can be embedded in <li> .

There are several elements in HTML that are quite special:

,

,

, , their sub-levels must is the specified element, the child level of

,

> must be the child level of <li>;<dl></dl> The level must be <dt></dt> or <dd></dd>; the sublevel of <table> must be <code><caption></caption> or <thead>, </thead> <tfoot>, </tfoot> <tbody>, etc., and the next sub-layer must be <code><tr> (<code><tr> Only exists in <code><thead>, </thead> <tfoot>, </tfoot> <tbody>), and then the content can be placed <code><td> or <code>&lt ;th>.

Inline elements

In fact, among the inline elements, you can still distinguish them. There are several elements (<img alt="HTML element nesting problem" >, <input>, etc. ) are special, they can define width and height. Although in the IE browser, all elements can define width and height, this is IE's own standard, and not all browsers support it, W3C We call them replaced elements. In fact, they are what we often call inline blocks. Although these elements are inline, they have a certain block (width and height can be set). We can also give any element the css attribute display:inline-block.
Elements that have inline-block themselves should not!

For more HTML element nesting issues and related articles, please pay attention to the PHP Chinese website!