Usage of @ rule in CSS

This article mainly introduces a summary of the usage of some @ rules in CSS. It is the basic knowledge for introductory learning of CSS. Friends in need can refer to it

at-rule is a statement that provides execution for CSS Or instructions on how to behave. Each declaration begins with @, followed by an available keyword, which acts as an identifier to indicate what the CSS should do. This is a general syntax, although there are other syntax variations for each at-rule.
General rules

General rules follow the following syntax:

@[KEYWORD] (RULE);

@charset
This rule defines the Character set if the stylesheet contains non-ASCII characters (e.g: UTF-8). Note that the character set placed in the HTTP header will override the @charset rule

@charset "UTF-8";

@import
This rule indicates that the requested stylesheet, in this line, if the content is correct , an external CSS file will be introduced.

@import 'global.css';

Although popular CSS preprocessors all support @import, it should be noted that they work differently from native CSS: the preprocessor grabs the CSS files and processes them into a CSS File, for native CSS, each @import is an independent HTTP request.
@namespace
This rule is very useful for applying CSS to XML HTML (XHTML), because the XHTML element can be used as a selector in CSS.​

/* Namespace for XHTML */
        @namespace url(http://www.w3.org/1999/xhtml);
        /* Namespace for SVG embedded in XHTML */
        @namespace svg url(http://www.w3.org/2000/svg);

Nested rules

Nested rules contain additional subset declarations, some of which can only be used in specific situations.​

@[KEYWORD] {
          /* Nested Statements */
        }

@document
This rule specifies conditions for the style sheet: it can only be applied to specific pages. For example, we provide a URL and then customize the styles for this specific page. In other pages, these styles will be ignored.

@document 
          /* Rules for a specific page */
          url(http://css-tricks.com/),
          /* Rules for pages with a URL that begin with... */
          url-prefix(http://css-tricks.com/snippets/),
          /* Rules for any page hosted on a domain */
          domain(css-tricks.com),
          /* Rules for all secure pages */
          regexp("https:.*")
        {
          /* Start styling */
          body { font-family: Comic Sans; }
        }

@font-face
This rule allows loading custom fonts on web pages. There are varying degrees of support for custom fonts, but this rule accepts statements to create and provide these fonts.

@font-face {
          font-family: 'MyWebFont';
          src:  url('myfont.woff2') format('woff2'),
                url('myfont.woff') format('woff');
        }

@keyframes
Among many CSS properties, this rule is the basis for keyframe animation and allows us to mark the beginning and end of animation.

@keyframes pulse {
          0% {
            background-color: #001f3f;
          }
          100% {
            background-color: #ff4136;
          }
        }

@media
This rule contains conditional declarations that can be used to specify styles for specific screens. These declarations can include screen sizes, which can be useful in screen-adaptive styles.

/* iPhone in Portrait and Landscape */
        @media only screen 
          and (min-device-width: 320px) 
          and (max-device-width: 480px)
          and (-webkit-min-device-pixel-ratio: 2) {
            .module { width: 100%; }
        }

Or apply the style only when the document is printed

@media print {
        }

@page
This rule defines styles for individual pages that will be printed. In particular, it can set margins for page pseudo-elements: :first, :left and :right

@page :first {
          margin: 1in;
        }

@supports
This rule can test whether the browser supports a certain Features/functions that will apply specific styles to these elements if the conditions are met. A bit like Modernizr, but really CSS properties.

/* Check one supported condition */
        @supports (display: flex) {
          .module { display: flex; }
        }
        /* Check multiple conditions */
        @supports (display: flex) and (-webkit-appearance: checkbox) {
          .module { display: flex; }
        }

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