How to use css

CSS is an important part of web design. It can be used to control the style and layout of web pages. In web design, using CSS can make web pages look more beautiful, orderly and easy to read. This article will introduce the basic use of CSS.

1. What is CSS?

The full name of CSS is Cascading Style Sheets (cascading style sheets). It is a markup language used to change the display style of HTML or XML documents. It is a technology of web design. The basic function of CSS style sheets is to apply different attributes, such as fonts, colors, backgrounds, etc., to different web page elements to achieve different visual effects.

2. The basic syntax of CSS

The basic syntax of CSS consists of selectors, attributes and values. The selector is used to select the HTML element to which the style is applied. The attribute specifies the attribute to be set, and the value is the specific value of the attribute.

For example, if we want to set the color of a paragraph to red in an HTML document, we can use the following code:

p { color: red; }

p in the code is The selector means to select all paragraph elements, color is the attribute, which means to set the color, and red is the value, which means to set the color to red.

3. CSS selector

CSS selector is used to select the HTML elements that need to be styled. It can be selected based on the element's tag name, class, ID and other attributes.

1. Tag selector

Tag selector refers to selecting elements based on the tag name of HTML elements. For example, to select all paragraph elements, you can use the following code:

p { color: red; }

2. Class selector

The class selector refers to selecting elements based on the class attribute of HTML elements. For example, to select all elements with the class name "intro" Elements can use the following code:

.intro { color: red; }

3.ID selector

The ID selector refers to selecting based on the ID attribute of the HTML element Element, the ID attribute of an HTML element is unique, and there cannot be two identical ID attributes in a page. For example, to select the element with the id "intro", you can use the following code:

intro { color: red; }

4. CSS attributes and values

CSS attributes and values ​​are used to control the style of elements. CSS provides many attributes and values ​​that can be used to set different style effects.

1. Font attributes

Font attributes can be used to set the font, size, color and other attributes of text, for example:

font-family: Arial, Verdana, sans- serif; / Font type/
font-size: 16px; / Font size/
color: #333; / Font color/

2. Background attribute

The background attribute can be used to set the background color, picture, position, etc. of the element, for example:

background-color: #f8f8f8; / Background color/
background-image: url("bg.jpg"); / Background image/
background-repeat: no-repeat; / The background does not repeat /
background-position: center center; / Background position/

3. Border attribute

The border attribute can be used to set the border style of the element , thickness, color, etc., for example:

border-style: solid; / Border style/
border-width: 1px; / Border width/
border-color: #ccc; / Border color/

4. Size attribute

The size attribute can be used to set the width, height, maximum width and Maximum height, etc., for example:

width: 500px; / Element width/
height: 300px; / Element height/
max-width: 100%; / Maximum width of element/
max-height: 100%; / Maximum height of element/

5. CSS priority

When an element is selected by multiple selectors, the CSS priority determines which style is applied to it. The priorities of CSS from high to low are: !important, inline style, ID selector, class selector and tag selector. When the priorities are the same, later styles will override previous styles.

For example, if you have the following style sheet:

p { color: blue; }
.intro { color: red; }

intro { color: green; }

So if there is such an element:

This is a paragraph

Then its text color is green, that is, the ID selector has the highest priority, covering the styles of the class selector and label selector.

6. CSS Inheritance

Inheritance refers to the function that an element can inherit the style attributes of its parent element. This feature allows the style sheet to be greatly reduced, reducing the file size and improving readability. For example:

body { font-size: 14px; }
p { color: #333; }

In this way, in a p element, the font size will be 14px, and the color As #333, the p element inherits the font size of the body element.

7. How to introduce CSS

CSS can be introduced in three ways: inline, internal and external. Inline style is for a single element, while external and internal styles are used to define the style sheet of the entire HTML page.

1. Inline style

Inline style can directly use the style attribute in the HTML element to define the style, for example:

This is a paragraph

2. Internal style

Internal style refers to defining the CSS style sheet in the head tag of the HTML document , for example:

<style>
    p { color: red; }
</style>

3.外部式

外部式是指将CSS样式表定义在一个单独的.css文件中，然后在HTML文档中引入，例如：

<link rel="stylesheet" type="text/css" href="style.css">

以上三种引入方式中，外部式是推荐使用的方式。这种方式可以使CSS样式表单独存放，方便维护和修改，也可以提高页面加载速度。

总结：

CSS是网页设计中的重要组成部分，它可以用来控制网页中的样式和布局。CSS的基本语法由选择器、属性和值组成，选择器用来选择要应用样式的HTML元素，属性指定要设置的属性，值是属性的具体数值。当同一个元素被多个选择器选中时，CSS的优先级决定它应用哪一个样式。CSS有三种引入方式：内联式、内部式和外部式，而外部式是推荐使用的方式。

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