CSS, the full name of Cascading Style Sheets, is a language used to design web page styles. CSS can separate the content and presentation style in HTML documents, making the page more beautiful and more in line with the designer's requirements. This article will introduce the basic concepts, syntax and practical applications of CSS to help beginners get started.
1. Basic concepts of CSS
CSS consists of selectors and declarations. Selectors are used to specify the HTML elements to which styles are to be applied, while declarations define the styles to be applied. The following are some important basic concepts:
1. Selector
The selector is the code that indicates which HTML element will be styled. Selectors can be tag selectors, class selectors, ID selectors, etc., which are introduced below.
(1) Tag selector
The tag selector is the most commonly used selector, which matches the tag name of the element in the HTML document. For example, the following code will set the text color of all
elements to red: h1 {
color: red;
}
(2) Class selector
The class selector uses . plus the class name to match. For example, the following code will center the text of an element with class "text-center":
.text-center { text-align: center; }
(3) ID selector
The ID selector uses # plus the ID name to match. Each element in an HTML document can only have one ID, so there can only be one element matched by the ID selector. For example, the following code will set the background color of the element with the id "main" to white:
#main { background-color: white; }
2. Declaration
The declaration refers to the style description applied to the corresponding HTML element. A declaration consists of attributes and values. For example, the following code will set the text color of all
elements to red: h1 {
color: red;
}
Here, color is the attribute name, and red is its attribute value. Each statement needs to be written within curly braces and ended with a semicolon.
2. CSS syntax
The syntax of CSS is very simple, just like key-value pairs. Each CSS rule consists of a selector and a declaration block wrapped in curly braces, as shown below:
选择器 { 属性: 值; 属性: 值; }
In this rule, the selector refers to the HTML element to which CSS styles are applied, and the attributes and The value is the description of the CSS style itself. For easier reading, multi-line rules often use indentation and line breaks to organize code.
The following is some CSS code for applying a gradient background:
body { background: linear-gradient(to right, #00bfff, #007fff); }
This rule indicates that a gradient background is applied to the body element of the document. The style will appear in a gradient from left to right, from #00bfff (light blue) to #007fff (dark blue).
3. Application of CSS
In HTML documents, you can directly use inline styles or use external style sheets to apply CSS styles.
1. Inline style
Inline style attributes are directly embedded into HTML tags in the form of inline styles. For example, the following code will set the text color of the element to red:
<span>Hello, CSS!</span>
However, inline styles only apply to the specified HTML element and cannot be inherited by other elements. Therefore, it inevitably makes the HTML code more complex.
2. External style sheet
In order to avoid using inline styles to cause confusion in HTML code, we can use external style sheets. External style sheets are saved separately from HTML documents in the form of .css files, making page maintenance more convenient. The following is a simple example of an external style sheet:
/* 在style.css文件中定义以下规则 */ h1 { color: red; } .text-center { text-align: center; } #main { background-color: white; }
In this example, three rules are defined in the CSS file, corresponding to
, class "text-center" and id in HTML is the element of "main". To use this CSS file, we need to add the following tags to the HTML file: <!-- 在HTML中引用style.css文件 -->
<link>
In this way, the HTML document will automatically apply the styles defined in the rules.
4. Summary
CSS is a very important design language, which makes web pages more beautiful, easy to read and easy to maintain. Understanding the basic concepts, syntax and practical application of CSS is an essential foundation for web design. I hope this article can help you better master the use of CSS.
The above is the detailed content of How to use css. For more information, please follow other related articles on the PHP Chinese website!

TonavigateReact'scomplexecosystemeffectively,understandthetoolsandlibraries,recognizetheirstrengthsandweaknesses,andintegratethemtoenhancedevelopment.StartwithcoreReactconceptsanduseState,thengraduallyintroducemorecomplexsolutionslikeReduxorMobXasnee

Reactuseskeystoefficientlyidentifylistitemsbyprovidingastableidentitytoeachelement.1)KeysallowReacttotrackchangesinlistswithoutre-renderingtheentirelist.2)Chooseuniqueandstablekeys,avoidingarrayindices.3)Correctkeyusagesignificantlyimprovesperformanc

KeysinReactarecrucialforoptimizingtherenderingprocessandmanagingdynamiclistseffectively.Tospotandfixkey-relatedissues:1)Adduniquekeystolistitemstoavoidwarningsandperformanceissues,2)Useuniqueidentifiersfromdatainsteadofindicesforstablekeys,3)Ensureke

React's one-way data binding ensures that data flows from the parent component to the child component. 1) The data flows to a single, and the changes in the state of the parent component can be passed to the child component, but the child component cannot directly affect the state of the parent component. 2) This method improves the predictability of data flows and simplifies debugging and testing. 3) By using controlled components and context, user interaction and inter-component communication can be handled while maintaining a one-way data stream.

KeysinReactarecrucialforefficientDOMupdatesandreconciliation.1)Choosestable,unique,andmeaningfulkeys,likeitemIDs.2)Fornestedlists,useuniquekeysateachlevel.3)Avoidusingarrayindicesorgeneratingkeysdynamicallytopreventperformanceissues.

useState()iscrucialforoptimizingReactappperformanceduetoitsimpactonre-rendersandupdates.Tooptimize:1)UseuseCallbacktomemoizefunctionsandpreventunnecessaryre-renders.2)EmployuseMemoforcachingexpensivecomputations.3)Breakstateintosmallervariablesformor

Use Context and useState to share states because they simplify state management in large React applications. 1) Reduce propdrilling, 2) The code is clearer, 3) It is easier to manage global state. However, pay attention to performance overhead and debugging complexity. The rational use of Context and optimization technology can improve the efficiency and maintainability of the application.

Using incorrect keys can cause performance issues and unexpected behavior in React applications. 1) The key is a unique identifier of the list item, helping React update the virtual DOM efficiently. 2) Using the same or non-unique key will cause list items to be reordered and component states to be lost. 3) Using stable and unique identifiers as keys can optimize performance and avoid full re-rendering. 4) Use tools such as ESLint to verify the correctness of the key. Proper use of keys ensures efficient and reliable React applications.


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

WebStorm Mac version
Useful JavaScript development tools

MantisBT
Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.

ZendStudio 13.5.1 Mac
Powerful PHP integrated development environment

SublimeText3 Chinese version
Chinese version, very easy to use

PhpStorm Mac version
The latest (2018.2.1) professional PHP integrated development tool
