这个函数使用记忆。并且可以多次调用，而不会出现两次加载和运行相同样式表的冲突。而且，它不会在样式表实际加载之前解析。

const loadStyle = function () {
    let cache = {};
    return function (src) {
        return cache[src] || (cache[src] = new Promise((resolve, reject) => {
            let s = document.createElement('link');
            s.rel = 'stylesheet';
            s.href = src;
            s.onload = resolve;
            s.onerror = reject;
            document.head.append(s);
        }));
    }
}();

请注意函数表达式后面的括号()。

样式表的并行加载:

Promise.all([
    loadStyle('/style1.css'),
    loadStyle('/style2.css'),
    // ...
]).then(() => {
    // do something
})

您可以对动态加载脚本使用相同的方法。

如果你使用jquery:

$('head').append('<link rel="stylesheet" type="text/css" href="style.css">');

有一个通用的jquery插件，可以按需加载css和JS文件的同步和异步。 它还可以跟踪已经加载的内容:) 参见:http://code.google.com/p/rloader/

我想分享一种方法，不仅加载css，但所有的资产(js, css，图像)和处理onload事件的一堆文件。async-assets-loader。请看下面的例子:

<script src="https://unpkg.com/async-assets-loader"></script>
<script>
var jsfile = "https://code.jquery.com/jquery-3.4.1.min.js";
var cssfile = "https://cdnjs.cloudflare.com/ajax/libs/materialize/1.0.0/css/materialize.min.css";
var imgfile = "https://logos.keycdn.com/keycdn-logo-black.png";
var assetsLoader = new asyncAssetsLoader();
assetsLoader.load([
      {uri: jsfile, type: "script"},
      {uri: cssfile, type: "style"},
      {uri: imgfile, type: "img"}
    ], function () {
      console.log("Assets are loaded");
      console.log("Img width: " + assetsLoader.getLoadedTags()[imgfile].width);
    });
</script> 

根据async-assets-loader文档

下面是用于加载JS和/或CSS的完整代码

function loadScript(directory, files){
  var head = document.getElementsByTagName("head")[0]
  var done = false
  var extension = '.js'
  for (var file of files){ 
    var path = directory + file + extension 
    var script = document.createElement("script")
    script.src = path        
    script.type = "text/javascript"
    script.onload = script.onreadystatechange = function() {
        if ( !done && (!this.readyState ||
            this.readyState == "loaded" || this.readyState == "complete") ) {
            done = true
            script.onload = script.onreadystatechange = null   // cleans up a little memory:
            head.removeChild(script)  // to avoid douple loading
        }
  };
  head.appendChild(script) 
  done = false
 }
}

function loadStyle(directory, files){
  var head = document.getElementsByTagName("head")[0]
  var extension = '.css'
  for (var file of files){ 
   var path = directory + file + extension 
   var link = document.createElement("link")
   link.href = path        
   link.type = "text/css"
   link.rel = "stylesheet" 
   head.appendChild(link) 
 }
}

(() => loadScript('libraries/', ['listen','functions', 'speak', 'commands', 'wsBrowser', 'main'])) ();
(() => loadScript('scripts/', ['index'])) ();

(() => loadStyle('styles/', ['index'])) ();

下面是jQuery的元素创建方法(我的偏好)和回调onLoad的方法:

var css = $("<link>", {
  "rel" : "stylesheet",
  "type" :  "text/css",
  "href" : "style.css"
})[0];

css.onload = function(){
  console.log("CSS IN IFRAME LOADED");
};

document
  .getElementsByTagName("head")[0]
  .appendChild(css);