title: Javascript - add css style to element with class
body: <p>I want to add only one css style in JS. I don't want to include jQuery for only one thing.</p>
  
  <p>My code:</p>
  
  <pre><code>document.addEventListener('DOMContentLoaded', function() {
      if (navigator.userAgent.indexOf('Safari') != -1 &amp;&amp; navigator.userAgent.indexOf('Chrome') == -1) {
          var productAttr = document.getElementsByClassName('product-attributes');
          productAttr.style.top = "-90px";
      }
  });
  </code></pre>
  
  <p>The error from console is:</p>
  
  <blockquote>
    <p>TypeError: 'undefined' is not an object (evaluating 'productAttr.style.top = "-90px"')</p>
  </blockquote>
  
  <p>If I want change other styles f.e. opacity or color, I get the same problem.</p>
  
  <p>How can I fix this ?</p>
  
  <p>Thanks in advance for help.</p>