A null key is not allowed in a hash literal.

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        <!DOCTYPE html>
        <html lang="en">
        <head>
          <meta charset="UTF-8">
          <meta name="viewport" content="width=device-width, initial-scale=1.0">
          <title>Error: A null key is not allowed in a hash literal</title>
          <style>
            body {
              font-family: Arial, sans-serif;
              background-color: #f2f2f2;
            }
            
            .container {
              max-width: 800px;
              margin: 0 auto;
              padding: 20px;
            }
            
            .error {
              background-color: #ffe6e6;
              border: 1px solid #ff8080;
              border-radius: 5px;
              padding: 10px;
              margin-bottom: 20px;
            }
            
            .error-title {
              font-size: 24px;
              color: #ff4d4d;
              margin-bottom: 10px;
            }
            
            .error-description {
              font-size: 16px;
              margin-bottom: 10px;
            }
            
            .error-example {
              font-family: monospace;
              font-size: 14px;
              background-color: #fff;
              border: 1px solid #ccc;
              border-radius: 5px;
              padding: 10px;
            }
          </style>
        </head>
        <body>
          <div class="container">
            <div class="error">
              <h2 class="error-title">Error: A null key is not allowed in a hash literal</h2>
              <p class="error-description">This error occurs when you try to include a null key in a hash literal.</p>
              <div class="error-example">
                <code>
                  const myHash = { null: 'value' };
                </code>
              </div>
              <p class="error-description">In the above example, the null key is not allowed in the hash literal. Instead, you should use a valid key.</p>
              <p class="error-description">For example:</p>
              <div class="error-example">
                <code>
                  const myHash = { key: 'value' };
                </code>
              </div>
            </div>
          </div>
        </body>
        </html>

The provided HTML content is an example of how to format an error message for the query “a null key is not allowed in a hash literal” within a `

` element. It includes an explanation of the error and provides a code example with both the incorrect and correct usage.

CSS styles are used to enhance the appearance of the error message. The container class sets a maximum width, centers the content, and adds padding. The error class defines the background color, border, and padding for the error box. The error-title and error-description classes set the font size and color for the error message. The error-example class styles the code example within a monospace font, with a white background, and a border.

The main structure of the HTML content includes:
– An HTML doctype declaration and opening `` tag.
– The `` section with meta tags for character encoding and viewport settings, as well as a title for the error.
– The `