Learning about replaced elements and non-replaced elements_html/css

Learning about replaced elements and non-replaced elements_html/css_WEB-ITnose

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Release： 2016-06-24 11:39:12

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Learning of replaced elements and non-replaced elements

@(Element)[Miaotong]

Introduction

Elements are the basis of document structure , in CSS, each element generates a box (box, also translated as "box") that contains the content of the element. But different elements will be displayed differently, for example, div and span are different, and strong and p are also different. Different types are specified for different elements in the document type definition (DTD), which is one of the reasons why DTD is important to documents. According to the characteristics of the element itself, it can be divided into replaced elements and non-replaced elements. Non-replaced elements are not clearly defined in the W3C, but we can have replaced elements corresponding to non-replaced elements, so it can be understood as In addition to replaced elements, other elements are non-replaced elements.

Replacement element

Replacement element means that the browser determines the specific display content of the element based on its tags and attributes.
Replacement elements are elements whose content is not controlled by the CSS visual formatting model, such as img tags, embedded documents (iframes, etc.) or applets. These are called replacement elements. For example, the img element reads image information and displays it through the value of the src attribute. However, if you view the (x)html code, you cannot see the actual content of the image, and the content of the img element is usually replaced by the image specified by the src attribute. ; For example, the type attribute of the input element determines whether to display an input box, a radio button, etc. (x) img, input, textarea, select, object in html are all replacement elements. These elements have no actual content, that is, they are empty elements, for example:

<img src="girl.jpg"/> <input type="submit" name="submit" value="提交"/>

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The browser will display these elements based on the element's tag type and attributes. Replacement elements also generate boxes in their display. Therefore, replacement elements usually have intrinsic dimensions: an intrinsic width, an intrinsic height, and an intrinsic ratio. For example, a bitmap has a width and height specified in absolute units, as well as an inherent aspect ratio. On the other hand, other documents may not have inherent dimensions, such as a blank HTML document.

The CSS rendering model does not consider the rendering of replaced element content. These replacement elements are rendered independently of CSS. Object, video, textarea, and input are also replacement elements, and audio and canvas are replacement elements in certain situations. Objects inserted using the CSS content property are anonymous replacement elements.

Replacing elements can increase the height of the line box, but does not affect line-height. Content area height value = padding-top padding-bottom margin-top margin-bottom height.
To replace the element in the center, you can set line-height = height

<!DOCTYPE html><html lang="en"><head>	<meta charset="UTF-8">	<title>CSS</title>	<style>		*{			margin:0;			padding:0;		}		input{			height:50px;			line-height:50px;			padding:10px;			margin:20px;			background-color:#9deaf1;			text-align:center;		}	</style></head><body>	<input type="text" value="美女"></body>	</html>

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The page diagram is as follows:

At the same time, this When display: inline-block; padding is valid for up, down, left and right, and margin is valid for up, down, left and right.

Non-replacement elements

Non-replacement elements: (X) Most elements of HTML are non-replacement elements. They directly tell the browser the content and display it. For example:

	<p>p的内容</p>	<label>label的内容</label>；

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The browser will display this content directly.
Adding padding-top or padding-bottom to non-replaced elements in the line will not affect the height of the line box, but the height of the content area will change. Margin-top and margin-bottom have no effect on the line box. Adding left and right margins affects the horizontal positioning of non-replaced elements within the line.

For inline elements, set the left and right padding, and the left and right padding will be visible. When setting the top and bottom padding, you can see that the padding area increases after setting the background color. For inline non-replaced elements, the row height will not be affected and the parent element will not be stretched. For inline replacement elements, the parent element is expanded. Take a look at the demo for a better understanding:

<!DOCTYPE html><html lang="en"><head>	<meta charset="UTF-8">	<title>CSS</title>	<style>		*{			margin:0;			padding:0;		}		div.wrapper{			margin:0 auto;			background-color:#9deaf1;			width:400px;		}		div input{			padding:50px;			margin:50px;			border:2px solid green;			background-color:#5dc2f6;		}	</style></head><body>	<div class="wrapper">		<input type="text" value="美女"/>	</div>	</body>	</html>

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Page diagram:

Code:

<!DOCTYPE html><html lang="en"><head>	<meta charset="UTF-8">	<title>CSS</title>	<style>		*{			margin:0;			padding:0;		}		div.wrapper{			margin-top:50px;			margin-left:50px;			background-color:#9deaf1;			width:500px;		}		div strong{			padding:50px;			margin:50px;			border:2px solid green;			background-color:#5dc2f6;		}	</style></head><body>	<div class="wrapper">		<strong>hello world</strong>	</div>	</body>	</html>

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Schematic diagram:

The above demo proves that inline replacement of elements, such as input, and setting upper and lower padding can expand the parent element.
As for inline non-replacement elements, such as strong, setting upper and lower padding only expands the range, but cannot expand the parent element and will not affect line-height;

inline elements (to be precise, non- The inline element of replace) cannot set margin and padding in the vertical direction. This is stipulated by the CSS standard.
The regulations for margin are here: Please click
The regulations for padding are here: Please click

The reason why padding cannot be set is because the value of padding is calculated based on the width of the target element, and The width of inline, non-replace elements is undefined.

If there is something incorrect or needs correction, please let me know, thank you----Miaotong