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What are selector declarations and nesting? Declaration and nesting of CSS selectors

零下一度Original: 2017-04-22 13:10:522620browse

When using CSS selectors to control HTML tags, the attributes of each selector can be declared multiple times at a time, and the selector itself can also be declared multiple at the same time. Moreover, any form of selector is legal, and tag selectors, class selectors and ID selectors can all be declared collectively. When declaring CSS selectors, if the styles of some selectors are exactly the same or partially the same, you can use collective declaration to declare selectors with the same style at the same time.

<span style="font-size:24px;"><html>
	<head>
		<title>
			集体声明
		</title>
		<style type="text/css">
			<!--
			h1,h2,h3,h4,h5,p{
			color:purple;
			font-size:15px;
		}
			h2.special,.special,#one{
			text-decoration:underline;
		}
			-->
		</style>
	</head>
	
	<body>
		<h1>集体声明h1</h1>
		<h2 class="special">集体声明h2</h2>
		<h3>集体声明h3</h3>
		<h4>集体声明h4</h4>
		<h5>集体声明h5</h5>
		<p>集体声明p1</p>
		<p class="special">集体声明p2</p>
		<p id="one">集体声明p3</p>
	</body>
</html>
</span>

Global declaration

For the actual website, if you want all the tags in the page to use the same CSS style, but it is not If you want to declare collectively one by one, you can use the global declaration symbol * to declare.

<span style="font-size:24px;"><html>
	<head>
		<title>
			集体声明
		</title>
		<style type="text/css">
			<!--
			*{
			color:purple;
			font-size:15px;
		}
			h2.special,.special,#one{
			text-decoration:underline;
		}
			-->
		</style>
	</head>
	
	<body>
		<h1>集体声明h1</h1>
		<h2 class="special">集体声明h2</h2>
		<h3>集体声明h3</h3>
		<h4>集体声明h4</h4>
		<h5>集体声明h5</h5>
		<p>集体声明p1</p>
		<p class="special">集体声明p2</p>
		<p id="one">集体声明p3</p>
	</body>
</html>
</span>

It can be seen that the two declarations have exactly the same effect, but using global declarations will greatly reduce the code.

Nesting of selectors

In CSS selectors, you can use nesting to declare HTML tags in special positions .

For example, controlling the b tag in the p tag

<span style="font-size:24px;"><html>
	<head>
		<title>
			CSS选择器的嵌套声明
		</title>
		<style type="text/css">
			<!--
			p b{
			color:maroon;
			text-decortion:underline;
		}
			-->
		</style>
	</head>
	
	<body>
		<p>嵌套使<b>用CSS</b>标记的方法</p>
		嵌套之外的<b>标记</b>不生效
	</body>
</html>
</span>

Nested selectors are widely used, not just nested tags By itself, both category selectors and ID selectors can be nested.

The following code uses three levels of nesting. In fact, more levels of nesting are allowed, indicating that the b6c5a531a458a2e790c1fd6421739d1c tag of the .top category contains. The e388a4556c0f65e1904146cc1a846bee tag of the top1 category, which contains the 8e99a69fbe029cd4e2b854e244eab143 tag, declares the style.

<span style="font-size:24px;">td.top .top1 strong{
			font-size:16px;
		}
<td class="top">
	<p class="top1">
		其他内容<strong>CSS控制的部分</strong>其他内容
	</p>
</td>
</span>

The nesting of selectors can greatly reduce the declaration of class and id in CSS. Therefore, when building the HTML of the page, usually only define class or id for the outer tag. id. If the inner tag can be represented by nesting, the nesting method is used. Therefore, there is no need to define a new class or id. Only when the sub-tag cannot take advantage of this rule, a separate declaration is made.

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