HTML and CSS best practices.

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1. Self-Closing Tags: While HTML5 allows self-closing tags, it's preferable to include the closing slash (/) for consistency across different HTML versions. Update the self-closing tags in the code as follows:
   <input type="text" class="placeholder" placeholder="Add to your list..." />

2. Whitespace and Indentation: Consider adding consistent indentation and whitespace to enhance the code's readability and maintainability.

3. Use Meaningful Class Names: Consider using more descriptive and meaningful class names to improve code understanding. For example, instead of .flex, you can use .flex-container or .flex-wrapper to provide more context.

4. Consider organizing your styles in a logical order. For example, organizing can start with global styles (such as * and body), followed by container styles, and then specific element styles.

5. Adding comments to the CSS code can provide clarity and make it easier for others to understand your intentions and the purpose of specific styles.