What are replaceable elements? (with examples)

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Recently, I accidentally saw a friend in a group chat posting a screenshot of Toutiao’s front-end interview questions. There was only one question about HTML, as follows:

What is a replaceable element? and non-replaceable elements, what is their difference? And give examples.

The HTML questions in the front-end interview are the least, and they are not difficult. There are only a few of them over and over again. We have talked about the most classic HTML semantics before, and today we will take this opportunity to talk about replaceable elements.

Definition

What is a replaceable element? As the name suggests, it is the element that will be replaced. (Awkward laugh...

For example, a typical replaceable element img:

<img src=xxx.jpg>

We did not write anything in the img tag, then it Where does the content come from?

is the browser downloading the image given by the src attribute and replacing the img tag with the image resource, and the browser does not know it before downloading The width and height of the image. Therefore, the replaceable element is special. Its width and height are determined by the content it loads. (Of course, CSS can override its style)

Example

Still use the img tag to give a few examples:

<img src="https://avatars2.githubusercontent.com/u/17703242?s=460&v=4">

If the image is displayed on the page, it will be the width and height of the image itself.

## The #img element also supports width and height attributes:

<img width="80" height="80" src="https://avatars2.githubusercontent.com/u/17703242?s=460&v=4">

At this time, the element is displayed with a width and height of 80 pixels.

If we use CSS to override its style:

img {
  width: 60px;
  height: 60px;
}

The display of this element is 60 pixels.

MDN’s explanation

Read it After the above example, it will be easier for us to understand conceptual knowledge.

The display effect of replaceable element (replaced element) is not controlled by CSS To control. These elements are an external object, and the rendering of their appearance is independent of CSS. Simply put, their content is not affected by the style of the current document. CSS can affect the position of replaceable elements , but will not affect the content of the replaceable element itself.

Typical replaceable elements are

<iframe> <video> <embed> <img> , and some elements are only treated as replaceable elements under specific circumstances, such as <input> .

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