这将选择第二个LI元素。
document.querySelector("li:not([class])")
要么
document.querySelector("li:not(.completed):not(.selected)")
// select li which doesn't have a 'class' attribute...
console.log(document.querySelector("li:not([class])"))
// select li which doesn't have a '.completed' and a '.selected' class...
console.log(document.querySelector("li:not(.completed):not(.selected)"))
<ul id="tasks">
<li class="completed selected">One Task</li>
<li>Two Task</li>
</ul>
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