Summary of how to use CSS selectors

This time I will bring you a summary of how to use CSS selectors, and what are the precautions for using CSS selectors. The following is a practical case, let's take a look.

CSS selector

1.id selector #id{ }, "#id" selects the element
2. Class selector.class{ }, ".class name" selects Element
3.Tag selector p{ }, "tag name" selected element
4.Wildcard selector { },"" Select all elements
5. Combined selector:
Group selector E,F "," separated by commas, select E,F elements at the same time
Descendant selector E F separated by spaces , select all F elements under the E element (no matter how many levels the F element is nested, it will still be selected)
Direct sub-selector E > F ">" separates, selects the direct sub-element F under the E element, That is, the first-level sub-element F
adjacent sibling selector under the E element E F, " " separates the directly adjacent element F
after selecting the E element. Common phase Neighbor selectors E ~ F, "~" separates all sibling elements F that follow after selected E element
6. Pseudo-class selector L-V-H-A,:link,:visited, :hover,:active
7. Pseudo-element selector
E::first-line Selects the first line of the E element content
E::first-letter Selects the first letter of the E element content
E::before Insert content before the E element
E::after Insert content after the E element
Before and after are locations where additional content can be inserted and need to be used with the content attribute
8 .Attribute selector
input[type="text"] {
width:150px;
}

Priority of the selector

Css selector priority core: Each selector has its own priority. The more specific the scope, the higher the priority.
CSS priorities from high to low are:
1. Using !important after an attribute will override the element style defined anywhere on the page.
2. Inline style written on element tag as style attribute
3.id selector
4. Class selector
5. Pseudo-class selector
6. Attribute selector
7. Tag selector
8. Wildcard selector
9. Browser customization
When the CSS style rule consists of multiple selectors, the weight of the id selector is 1000. The class selector is 100 and the label selector is 10. Which one takes priority is determined by the sum of the weights. When the weights of two CSS rules are the same, which one is more specific will be used, that is, the selector with a higher weight will be more specific and have a higher priority. When the two selector rules and weights are the same, the later style will overwrite the previous one!
p {color: #333;}
p {color: #666;}
In this way, the color of the p copy will obviously be #666

class and id usage scenarios

id is a unique identifier on the page, and class identifies a certain type of style on the page. It is universal and can be used repeatedly. The class name of an element can be written as class="intro other", that is, there can be multiple class names, which means superimposing the styles corresponding to the two class names. The id name cannot be written like this. ID names are often used in page layouts (marking large frames), and classes are generally used in local page layouts for style definition. Because the class names can be written the same when naming, you only need to write the style once for the class, and it can be used by everyone. Reuse elements of the same style. Delimit appropriate namespaces when using CSS selectors to improve code readability and facilitate maintenance

Examples of selector usage

html
#header{} /*选中id为header的元素*/
.header{}    /*选中class=header的元素*/ 
.header .logo{}  /*选中class=header下的所有class=logo的元素*/ 
.header.mobile{}  /*选中class="header mobile"的元素*/ 
.header p, .header h3{}  /*选中class=header元素下的所有p元素，同时选中class=header元素下的所有h3元素*/ 
#header .nav>li{}  /*选中id=header元素下的所有class=nav元素的直接子元素li*/ 
#header a:hover{}  /*选中id=header元素下的所有a元素，并使用hover伪类*/

Common pseudo-class selectors

【1】Structural pseudo-class selectors
E: first-child selects the first child element under the parent element where E is located, and the child element is the E element
E:last-child selects the last child element under the parent element where E is located, and the child element is the E element
E:root selects the element of the root node where E is located. For HTML, it selects the HTML element.
E:nth-child(n) selects the nth child element under the parent element where E is located, and the child element is the E element.
E:nth-last-child(n) Selects the nth child element from the bottom under the parent element where E is located, and the child element is the E element
E:nth-of-type(n) Selects the parent element where E is located The nth E element among the elements of the same type under the element
E:nth-last-of-type(n) Selects the nth E element from the bottom among the elements of the same type under the parent element where E is located
E:first-of-type Selects the first E element among the elements of the same type under the parent element where E is located
E:last-of-type Selects the last E element among the elements of the same type under the parent element where E is located
E:only-child matches the only child element within the parent element, which is equivalent to: first-child:last-child or:nth-child(1):nth-last-child(1)
E :only-of-type matches the only child element using the same tag under the parent element, which is equivalent to :first-of-type:last-of-type or :nth-of-type(1):nth-last-of -type(1)
E:empty matches an element that has no child elements, and the element does not have any text nodes
E:not(F) matches any element that does not match the current selector
[2]Dynamic pseudo-class selector Sequence L-V-H-A
link-visited-hover-active

a:link{
 color:red;
 }
 
 a:visited{
 color:blue;
 }
 
 a:hover{
 color:gree;
 font-size:20px;
 }
 
 a:active{
 color:gold;
 }
 a:focus{
 color:gold; //a元素获得焦点后的样式
 }

The role and difference between:first-child and:first-of-type

E:first-child specifies the element E and finds the first E element under its parent element
E: first-of-type specifies the element of E type and finds the E type under its parent element The first

code example of the element:

html
<style>
.item1:first-child{ color: red;}
.item1:first-of-type{ background: blue;}
</style> 
<p> 
</p><p>aa</p>
<h3 id="bb">bb</h3>
<h3 id="ccc">ccc</h3> 

.item1:first-child{color :red;}class=The first child element item1 font under the parent element p of the item1 element is red<h3 id="bb-h-h-ccc-h">bb<h3>,<h3>ccc<h3></h3> </h3> </h3> </h3>Although class =item1 but they are not the first child element under its parent element.

.item1:first-of-type{background:blue;}The first class=item1 element among the elements of the same type under the parent element of the class=item1 element. <p class="item1">aa</p> Select the first element (p, h3) of the same type under the parent element p, namely aa, bb, with a blue background.

text-align: The role of center

Sets the horizontal center alignment of the text within the element. text-align is applied to block-level elements (p or p). The alignment of the internal inline elements (text, pictures, input boxes) of this block-level element (p/p) can be set.
text-align has 5 values: left/right/center/justify/inherit, left-aligned/right-aligned/center-aligned/aligned at both ends/inherit parent element align value. When justifying both ends, the word spacing in each line may be inconsistent.

I believe you have mastered the method after reading the case in this article. For more exciting information, please pay attention to other related articles on the php Chinese website!

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