PROSAGA码农传奇-CDN-如何正确导入CDN，包含CSS和jQuery CDN的代码有什么问题？

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  <P>
    这是您的问题的解决方案，只需将“ref”更改为“rel”，您就可以开始了。
一切都是正确的，因为你的外部样式表因“ref”而无法加载。要加载任何外部css文件，请使用“rel”而不是“ref”。我附上了一段代码供您参考。我希望，它解决了你的问题。
  </p>
  <P>
  </p>
  <div class =“snippet”data-lang =“js”data-hide =“false”data-console =“true”data-babel =“false”>
    <div class =“snippet-code”>
       <pre class="snippet-code-html lang-html prettyprint-override">
        <code>
          
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css">
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.0.13/css/all.css">

.pricing .card {
  border: none;
  border-radius: 1rem;
  transition: all 0.2s;
  box-shadow: 0 0.5rem 1rem 0 rgba(0, 0, 0, 0.1);
}

.pricing hr {
  margin: 1.5rem 0;
}

.pricing .card-title {
  margin: 0.5rem 0;
  font-size: 0.9rem;
  letter-spacing: .1rem;
  font-weight: bold;
}

.pricing .card-price {
  font-size: 3rem;
  margin: 0;
}

.pricing .card-price .period {
  font-size: 0.8rem;
}

.pricing ul li {
  margin-bottom: 1rem;
}

.pricing .text-muted {
  opacity: 0.7;
}

.pricing .btn {
  font-size: 80%;
  border-radius: 5rem;
  letter-spacing: .1rem;
  font-weight: bold;
  padding: 1rem;
  opacity: 0.7;
  transition: all 0.2s;
}

/* Hover Effects on Card */

@media (min-width: 992px) {
  .pricing .card:hover {
    margin-top: -.25rem;
    margin-bottom: .25rem;
    box-shadow: 0 0.5rem 1rem 0 rgba(0, 0, 0, 0.3);
  }
  .pricing .card:hover .btn {
    opacity: 1;
  }
}

</style>

<section class="pricing py-5">
    <div class="container">
      <div class="row">
        
        <div class="col-lg-4">
          <div class="card mb-5 mb-lg-0">
            <div class="card-body">
              <h5 class="card-title text-muted text-uppercase text-center">Free</h5>
              <h6 class="card-price text-center">$0<span class="period">/month</span></h6>
              <hr>
              <ul class="fa-ul">
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Single User</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>5GB Storage</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Unlimited Public Projects</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Community Access</li>
                <li class="text-muted"><span class="fa-li"><i class="fas fa-times"></i></span>Unlimited Private Projects</li>
                <li class="text-muted"><span class="fa-li"><i class="fas fa-times"></i></span>Dedicated Phone Support</li>
                <li class="text-muted"><span class="fa-li"><i class="fas fa-times"></i></span>Free Subdomain</li>
                <li class="text-muted"><span class="fa-li"><i class="fas fa-times"></i></span>Monthly Status Reports</li>
              </ul>
              <a href="#" class="btn btn-block btn-primary text-uppercase">Button</a>
            </div>
          </div>
        </div>
        
        <div class="col-lg-4">
          <div class="card mb-5 mb-lg-0">
            <div class="card-body">
              <h5 class="card-title text-muted text-uppercase text-center">Plus</h5>
              <h6 class="card-price text-center">$9<span class="period">/month</span></h6>
              <hr>
              <ul class="fa-ul">
                <li><span class="fa-li"><i class="fas fa-check"></i></span><strong>5 Users</strong></li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>50GB Storage</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Unlimited Public Projects</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Community Access</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Unlimited Private Projects</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Dedicated Phone Support</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Free Subdomain</li>
                <li class="text-muted"><span class="fa-li"><i class="fas fa-times"></i></span>Monthly Status Reports</li>
              </ul>
              <a href="#" class="btn btn-block btn-primary text-uppercase">Button</a>
            </div>
          </div>
        </div>
        
        <div class="col-lg-4">
          <div class="card">
            <div class="card-body">
              <h5 class="card-title text-muted text-uppercase text-center">Pro</h5>
              <h6 class="card-price text-center">$49<span class="period">/month</span></h6>
              <hr>
              <ul class="fa-ul">
                <li><span class="fa-li"><i class="fas fa-check"></i></span><strong>Unlimited Users</strong></li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>150GB Storage</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Unlimited Public Projects</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Community Access</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Unlimited Private Projects</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Dedicated Phone Support</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span><strong>Unlimited</strong> Free Subdomains</li>
                <li><span class="fa-li"><i class="fas fa-check"></i></span>Monthly Status Reports</li>
              </ul>
              <a href="#" class="btn btn-block btn-primary text-uppercase">Button</a>
            </div>
          </div>
        </div>
      </div>
    </div>
  </section>