本文作者为奇舞团前端开发工程师 李慧娟
引言
最近看了2022年全球CSS调查报告，发现了一些不常见的伪类和伪元素。
伪类
:has()
html
<div>
<h1>H1</h1>
<h2>H2</h2>
<p>h1{margin: 0 0 0.25rem 0}</p>
</div>
css
h1, h2 {
margin: 0 0 1.0rem 0;
}
h1:has(+ h2) { // h1后面相邻兄弟元素是h2时
margin: 0 0 0.25rem 0;
}
:is()
html
<div>
<h1>H1</h1>
<h2>H2</h2>
<h3>H3</h3>
<h4>H4</h4>
<p>h1、h2和h3 {margin: 0 0 0.25rem 0}</p>
</div>
css
h1, h2, h3 {
margin: 0 0 1.0rem 0;
}
:is(h1, h2, h3):has(+ :is(h2, h3, h4)) { // h1 or h2 or h3 后面相邻兄弟是 h2 or h3 or h4 时
margin: 0 0 0.25rem 0;
}
:host
html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>:host</title>
</head>
<body>
<my-component></my-component>
<script>
let shadow = document.querySelector("my-component").attachShadow({ mode: "open"})
let styleEle = document.createElement("style")
styleEle.textContent = `
:host{
display: block;
margin: 20px;
width: 500px;
height: 300px;
border: 3px solid green;
}
:host div {
font-size: 30px;
border: 2px solid blue;
}
`
let headerEle = document.createElement("div");
headerEle.innerText = "选取内部使用该部分 CSS 的 Shadow host 元素，其实也就是自定义标签元素。(注意：:host 选择器只在 Shadow DOM 中使用才有效果。)";
shadow.appendChild(headerEle);
shadow.appendChild(styleEle);
</script>
</body>
</html>
:host()
html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>:host()伪类函数</title>
</head>
<body>
<my-component class="my-component"></my-component>
<my-component></my-component>
<script>
class MyComponent extends HTMLElement {
constructor () {
super();
this.shadow = this.attachShadow({mode: "open"});
let styleEle = document.createElement("style");
styleEle.textContent = `
:host(.my-component){
display: block;
margin: 20px;
width: 300px;
height: 200px;
border: 3px solid green;
}
:host .component-header{
border: 2px solid yellow;
padding:10px;
background-color: grey;
font-size: 16px;
font-weight: bold;
}
`;
this.shadow.appendChild(styleEle);
let headerEle = document.createElement("div");
headerEle.className = "component-header";
headerEle.innerText = ":host() 的作用是获取给定选择器的 Shadow Host。(注：:host() 的参数是必传的，否则选择器函数失效)";
this.shadow.appendChild(headerEle);
}
}
window.customElements.define("my-component", MyComponent);
</script>
</body>
</html>
:host-textContent()
html
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>:host-textContent()伪类函数</title>
</head>
<body>
<div id="container">
<my-component></my-component>
</div>
<my-component></my-component>
<script>
class MyComponent extends HTMLElement {
constructor () {
super();
this.shadow = this.attachShadow({mode: "open"});
let styleEle = document.createElement("style");
styleEle.textContent = `
:host-context(#container){
display: block;
margin: 20px;
width: 300px;
height: 200px;
border: 3px solid green;
}
:host .component-header{
border: 2px solid yellow;
padding:10px;
background-color: #4caf50;
font-size: 16px;
font-weight: bold;
}
`;
this.shadow.appendChild(styleEle);
let headerEle = document.createElement("div");
headerEle.className = "component-header";
headerEle.innerText = ":host-context() 用来选择特定祖先内部的自定义元素，祖先元素选择器通过参数传入。(注：:host-context() 的参数是必传的，否则选择器函数失效)";
this.shadow.appendChild(headerEle);
}
}
window.customElements.define("my-component", MyComponent);
</script>
</body>
</html>
效果：
:in-range & :out-of-range
html
<style>
input {
width: 180px;
height: 32px;
color: white;
font-weight: bold;
font-size: 20px;
}
input:in-range {
background: yellowgreen;
}
input:out-of-range {
background-color: brown;
}
</style>
<p>:in-range 输入的值在指定区间内时，设置指定样式</p>
<p>:out-of-range 输入的值在指定区间外时，设置指定样式</p>
<input type="number" min="5" max="10" value="7" />
<input type="number" min="5" max="10" value="3" />
效果：
:root
html
<style>
:root {
--green: #9acd32;
--font-size: 14px;
--font-family-sans-serif: -apple-system, BlinkMacSystemFont;
}
body {
color: var(--green);
font-size: var(--font-size);
font-family: var(--font-family-sans-serif);
}
</style>
<div>
:root中声明相当于全局属性，页面可以使用var()来引用
</div>
##伪元素
::first-letter
html
<style>
p::first-letter {
font-size: 1.5rem;
font-weight: bold;
color: brown;
}
</style>
<p>Scientists exploring the depths of Monterey Bay unexpectedly encountered a rare and unique species of dragonfish. This species is the rarest of its species.</p>
<p>中文字体时.</p>
结果：
::placeholder 设置placeholder的样式
::selection 鼠标选中文本的样式
##总结
- :host 范围最大，匹配所有的自定义元素实例；
- :host() 只选择自身包含特定选择器的自定义元素；
- :host-context() 选择拥有特定选择器父元素的自定义元素。
##参考资料 mdn web docs
