问如何检查css类是否在网站中实现？

EN

function isDefined(selector) {
  for (var i = 0; i < document.styleSheets.length; i++) {
    var sheet = document.styleSheets[i];
    for (var j = 0; j < sheet.cssRules.length; j++) {
      return sheet.cssRules[j].selectorText == selector;
    }
  }
}

var styleElement = document.createElement("style");
styleElement.type = 'text/css';
document.head.appendChild(styleElement);

var myStylesheet = document.styleSheets[document.styleSheets.length - 1];

if (!isDefined("p")) {
  myStylesheet.insertRule("p { background-color: yellow; }")
}

if (!isDefined("h1")) {
  myStylesheet.insertRule("h1 { background-color: yellow; }")
}

p {
  font-size: 12px;
}

<h1>hello</h1>
<p>world</p>