https://www.w3schools.com/css/css_selectors.asp
A CSS selector selects the HTML element(s) you want to style.
CSS Selectors
CSS selectors are used to "find" (or select) the HTML elements you want to style.
We can divide CSS selectors into five categories:
- Simple selectors (select elements based on name, id, class)
- Combinator selectors (select elements based on a specific relationship between them)
- Pseudo-class selectors (select elements based on a certain state)
- Pseudo-elements selectors (select and style a part of an element)
- Attribute selectors (select elements based on an attribute or attribute value)
This page will explain the most basic CSS selectors.
The CSS element Selector
The element selector selects HTML elements based on the element name.
Example
Here, all <p> elements on the page will be center-aligned, with a red text color:
p {text-align: center;
color: red;
}
The CSS id Selector
The id selector uses the id attribute of an HTML element to select a specific element.
The id of an element is unique within a page, so the id selector is used to select one unique element!
To select an element with a specific id, write a hash (#) character, followed by the id of the element.
Example
The CSS rule below will be applied to the HTML element with id="para1":
#para1 {text-align: center;
color: red;
}
Note: An id name cannot start with a number!
The CSS class Selector
The class selector selects HTML elements with a specific class attribute.
To select elements with a specific class, write a period (.) character, followed by the class name.
Example
In this example all HTML elements with class="center" will be red and center-aligned:
.center {text-align: center;
color: red;
}
You can also specify that only specific HTML elements should be affected by a class.
Example
In this example only <p> elements with class="center" will be red and center-aligned:
p.center {text-align: center;
color: red;
}
HTML elements can also refer to more than one class.
Example
In this example the <p> element will be styled according to class="center" and to class="large":
<p class="center large">This paragraph refers to two classes.</p>Note: A class name cannot start with a number!
The CSS Universal Selector
The universal selector (*) selects all HTML elements on the page.
Example
The CSS rule below will affect every HTML element on the page:
* {text-align: center;
color: blue;
}
The CSS Grouping Selector
The grouping selector selects all the HTML elements with the same style definitions.
Look at the following CSS code (the h1, h2, and p elements have the same style definitions):
h1 {text-align: center;
color: red;
}
h2 {
text-align: center;
color: red;
}
p {
text-align: center;
color: red;
}
It will be better to group the selectors, to minimize the code.
To group selectors, separate each selector with a comma.
Example
In this example we have grouped the selectors from the code above:
h1, h2, p {text-align: center;
color: red;
}
All CSS Simple Selectors
| Selector | Example | Example description |
|---|---|---|
| #id | #firstname | Selects the element with id="firstname" |
| .class | .intro | Selects all elements with class="intro" |
| element.class | p.intro | Selects only <p> elements with class="intro" |
| * | * | Selects all elements |
| element | p | Selects all <p> elements |
| element,element,.. | div, p | Selects all <div> elements and all <p> elements |
Video: CSS Simple Selectors
标签：HTML,css03,center,element,elements,Selectors,class,CSS From： https://www.cnblogs.com/emanlee/p/18218801