# @规则
[`@charset`](https://developer.mozilla.org/en-US/docs/Web/CSS/@charset) — Defines the character set used by the style sheet.
[`@import`](https://developer.mozilla.org/en-US/docs/Web/CSS/@import) — Tells the CSS engine to include an external style sheet.
[`@namespace`](https://developer.mozilla.org/en-US/docs/Web/CSS/@namespace) — Tells the CSS engine that all its content must be considered prefixed with an XML namespace.
## 嵌套 @规则
`@media`— 满足媒体查询条件则里面的 CSS 生效。例如：Bootstrap 使用下面的范围作为条件
```css
/* https://getbootstrap.com/docs/4.2/layout/overview/#responsive-breakpoints */
// Extra small devices (portrait phones, less than 576px)
// No media query for `xs` since this is the default in Bootstrap
// Small devices (landscape phones, 576px and up)
@media (min- 576px) { ... }
// Medium devices (tablets, 768px and up)
@media (min- 768px) { ... }
// Large devices (desktops, 992px and up)
@media (min- 992px) { ... }
// Extra large devices (large desktops, 1200px and up)
@media (min- 1200px) { ... }
```
`@font-face`— 可以给文本指定的自定义字体。例如：antd 用她指定中文的单双引号的字体
```css
/*
https://github.com/ant-design/ant-design/blob/3.12.1/components/style/core/base.less
指定中文的单双引号的字体
*/
@font-face {
font-family: "Chinese Quote";
src: local("PingFang SC"), local("SimSun");
unicode-range: U+2018, U+2019, U+201c, U+201d;
}
```
`@support`— 被检查的 CSS 声明有效则里面的 CSS 生效（IE11 不支持这个规则）。例如：为支持 flex 的浏览器提供 flex 的样式
```css
@supports (display: flex) {
// code for supporting browsers
}
```
等。。
# 参考
[@规则 | MDN](https://developer.mozilla.org/zh-CN/docs/Web/CSS/At-rule)