How to use @ rule in CSS? Usage introduction

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at-rule is a statement that provides execution 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:

The code is as follows:

@[KEYWORD] (RULE);

@charset

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

The code is as follows:

@charset "UTF-8";

@import

This rule indicates Request the style sheet. In this line, if the content is correct, an external CSS file will be introduced.

The code is as follows:

@import 'global.css';

Although popular CSS preprocessors support @import, it should be noted that their working principles are different from native CSS: the preprocessor will crawl CSS files and process them into a CSS file. For native CSS, each @import is a separate 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 use.

The code is as follows:

/* 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 for specific Condition.

The code is as follows:

@[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.

The code is as follows:

@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, and there are differences for custom fonts level of support, but this rule accepts statements to create and provide these fonts.

The code is as follows:

@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 of keyframe animation and allows us to mark animations Start and end signs.

The code is as follows:

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

@media

This rule contains conditional statements that can be used to specify styles for specific screens. These statements can include screen sizes. , it will be very useful in screen-adaptive styles.

The code is as follows:

/* 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 only use the style when printing the document

The code is as follows:

@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

The code is as follows:

@page :first {
    margin: 1in;
}

@supports

This rule can test whether the browser supports a certain feature/function. If the conditions are met, specific styles will be applied to these elements. A bit like Modernizr, but really CSS properties.

The code is as follows:

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

Summary

at-rule can make CSS do some crazy and interesting things. Although the examples in the article are basic, you can see how they can be used with styles for specific conditions to create user experiences and interactions that match specific scenarios.

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