Cross-Browser Incompatibility- To make your application as cross-browser friendly as possible, test it on at least three major browsers, such as Chrome, Firefox, Edge, and Safari

Incomplete Input Validation- Most beginner developers perform input validation just on the client side. Your app will be vulnerable to hacker attacks such as Session Hijacking if you haven't done server-side validations. A SSL certificate is required

Image Optimization Ignored- Image optimization is critical for lowering your Web app's bandwidth utilization. Instead of JPG or PNG, use SVGs for logos, icons, and simple pictures

Writing CSS and JS in HTML Files- This is one of the most common mistakes, but developers continue to make it. It's a poor habit that will make your code unreadable and difficult to test and debug. Security flaws and a slew of other issues are all possibilities.

Not developing for varied screen sizes- The development of smartphones with various screen resolutions has resulted in one of many new methods to view online information, as well as several new web development difficulties. The program must be optimized for multiple screen sizes and angles

Overusing Libraries- Using a large number of libraries in your application will raise the package size and build time. This will eventually increase the time it takes for a page to load