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471 lines
7.3 KiB
Markdown
471 lines
7.3 KiB
Markdown
---
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title: CSS · 基础笔记
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date: 2021-12-06T11:59:47+08:00
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slug: css-fundamental
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tags:
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- CSS
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lastmod: 2022-05-07T05:30:27.910Z
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---
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一些随堂笔记。
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课程：[Build Responsive Real-World Websites with HTML and CSS](https://www.udemy.com/course/design-and-develop-a-killer-website-with-html5-and-css3/)
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## css 是什么
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- **C**ascading**S**tyle**S**heets (层叠式样式表）
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- CSS describes the **visual style and presentation** of the **content written in HTML**
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- CSS consists of countless **properties** that developers use to format the content: properties about font, text, spacing, layout, etc.
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## 分类
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### inline CSS
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```html
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<p style="color:blue">text</p>
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```
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> 最好不用
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### internal CSS
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放在`<head>`里面的`<style>`，如：
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```html
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<head>
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<style>
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h1 {
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color: blue;
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}
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</style>
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</head>
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```
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- 代码很长的时候，整理很麻烦
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### external CSS
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- 引用单独的 css 文件，如 `style.css`
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- 放在`<head>`里用`<link>`引用，如：
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```html
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<head>
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<link href="style.css" ref="stylesheet" />
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</head>
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```
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关于`<link>` ，可参考:[MDN](https://developer.mozilla.org/zh-CN/docs/Web/HTML/Element/link#attr-rel)
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## 组成
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## Selectors
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### descendent selector
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```css
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footer p {
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font-family: sans;
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}
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article header p {
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color: blue;
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}
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```
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### line selector
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```css
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h1,
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h2,
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h3 {
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color: blue;
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}
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```
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定义特定元素样式的两种方式：CSS ID、class attributes
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## id
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> 给每个元素一个 id，仅能用一次，尽量不要用
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在 HTML 中：
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```html
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<p id="author">text</p>
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```
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在 CSS 中：
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```css
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#author {
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font-family: sans;
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}
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```
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### class attributes
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> 能无限复用
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在 HTML 中：
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```html
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<p class="author">text</p>
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```
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在 CSS 中：
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```css
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.author {
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font-family: sans;
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}
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```
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> 能无限复用
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在 HTML 中：
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```html
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<p class="author">text</p>
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```
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在 CSS 中：
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```css
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.author {
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font-family: sans;
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}
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```
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### universal selector
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- 对所有元素生效
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- 优先级最低
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- 不可继承
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```css
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* {
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color: #1098ab;
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}
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```
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### body
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- 在`<body>` 里的通常只应用于文本元素（text）
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- Not all properties get inherited. It’s mostly ones **related to text**: `font-family`, `font-size`, `font-weight`, `font-style`, `color`, `line-height`, `letter-spacing`, `text-align`, `text-transform`, `text-shadow`, `list-style`, etc.
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- 可继承
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### 优先级
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#### conflicting between selectors
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- 尽量不要用`!important`
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```css
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foot p {
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color: green !important;
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}
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```
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- 所有 selector 都会被应用，只有冲突的按优先级应用
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- 当使用相同类别的 selector 时，应用最后一个
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#### inheritance
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- inheriting elements can easily be override
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- 优先级最低
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- body 可继承，universal selector 不可以
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## 颜色
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### RGB/RGBA
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- 基本`(r,g,b,alpha)`
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- 白色`(255,255,255)`
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- 黑色`(0,0,0)`
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## Hexadecimal Colors
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- 0 to ff (255 in hexadecimal numbers):
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`#00ffff`
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- Shorthand, when all colors are identical pairs
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`#off`
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当需要透明度的时候才用 rgb 颜色，一般用 hex
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## Pseudo Class
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> 用来指定特定元素
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### 第一个元素/最后一个元素
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```css
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li:first-child {
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font-weight: bold;
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}
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li:last-child {
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font-style: italic;
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}
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```
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### 奇数/偶数 /特定次序
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```css
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li:nth-child(odd) {
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font-style: italic;
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}
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li:nth-child(even) {
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font-style: italic;
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}
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li:nth-child(3) {
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font-style: italic;
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}
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```
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### 多种元素时
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如下所示，当 HTML 中`<p>`并不是`<article>`里面的第一个元素时，不生效。
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即当母元素(parent element)里有多种元素时(child elements)，不宜使用伪类(pseudo class)，可以在列中使用，如`<li>`
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```html
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<article>
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<head></head>
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<p></p>
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</article>
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```
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```css
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article p:fist-child {
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font-family: sans;
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}
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```
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### Style hyperlinks
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四个状态都应定义，并按顺序排列
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#### link
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不进行交互的预览下
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```css
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a:link {
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color: #1098ad;
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}
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```
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#### visited
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点击后
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```css
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a:visited {
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color: #777;
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}
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```
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#### hover
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悬停时
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```css
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a:hover {
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color: orangered;
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font-weight: bold;
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text-decoration: underline dotted orangered;
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}
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```
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#### active
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点击时(通常和 hover 同时出现
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```css
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a:active {
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background-color: black;
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font-style: italic;
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}
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```
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## Pseudo Elements
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> any pseudo elements is actually an inline element
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在 HTML 中并不真实存在，但仍可在 CSS 中选择，如行内第一个字符：
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在 HTML 中并不真实存在，但仍可在 CSS 中选择，如行内第一个字符：
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```css
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h1::first-letter {
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font-style: normal;
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}
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```
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段内第一行：
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```css
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p::first-line {
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color: red;
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}
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```
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### adjacent sibiling selector
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在同一 parent element 里，下面最临近的元素
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如 h3 标题下的 p：
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```css
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h3 + p::first-line {
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color: red;
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}
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```
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### after
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- 就算没有文字内容，也要定义`content`
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- 位于行末
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```css
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h2::after {
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content: 'TOP';
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font-size: 16px;
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font-weight: bold;
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color: black;
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padding: 5px 15px;
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display: inline-block;
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position: absolute;
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top: -10px;
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right: -25px;
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background-color: #ffe70e;
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}
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h2 {
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position: relative;
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}
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```
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### before
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- 位于行前
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```css
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h2::before {
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}
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```
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## Box Model
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### Type of boxes:
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#### inline boxes
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#### block-level boxes
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#### inline-block boxes
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img 其实是 inline-block box
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### padding
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内边距
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```css
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padding: 上下 左右;
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```
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#### reset margin ane padding
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```css
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* {
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margin: 0;
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padding: 0;
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}
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```
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不要用 body
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### margin
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外边距
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#### collapsing margins
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- 当两元素的 margin 重合时，大的会覆盖小的
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### Dimensions
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#### width
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- percentage: percentage of the width of the parents containers
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## position
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### Normal Flow
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- 默认状态
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### Absolute Position
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- 绝对位置
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- 常用于一些小元素
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- parent elements/containers should set to `relative`
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- 取决于最近层次的 relative parent elements/containers
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### centering page
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use the `<div>` element to create a container class, then set the `margin-left` and `margin-right` to `auto`
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```css
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.container {
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width: 800px;
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margin: 0 auto;
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}
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```
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## 基本操作
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### CSS 注释
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- `/*需要注释的内容*/`
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- VS code 快捷键 :`CMD+/` (mac) `Control+/` (win)
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- 调出开发者工具`cmd+i`
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### Fix bugs
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- 方法：
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- **inspect**
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- 检查刚开始变得不一样的地方
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- 如果有很多 selector，复杂的那个会被首先应用
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- 检查 css 文件引用链接是否正确
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- 工具：
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HTML 校对器:
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[Markup Validation Service](https://validator.w3.org/)
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代码比对:
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[Diffchecker](https://www.diffchecker.com/)
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