Custom HTML Course Application Form

I need a responsive course application form built from scratch with clean HTML5, CSS3 and vanilla JavaScript—no WordPress, no Google Forms. The form should gather the usual applicant details (name, contact info, course choice, and any extra fields I may add later) and give clear, inline validation messages so mistakes are caught before submission.

Keep the code modular: place the action URL or fetch/AJAX call in one well-commented section so I can plug in my own back-end or an email service like Formspree without digging through the markup. Light helper frameworks such as Bootstrap or Tailwind are fine if they stay lightweight; steer clear of anything heavy.

Deliverables
• A fully responsive .html file containing the form markup
• Separate, well-commented .css and .js files
• Client-side validation for required fields with friendly error messaging
• Success/error feedback after submission
• A brief README explaining file structure, how to adjust fields or rules, and where to swap the submission endpoint

Acceptance criteria
• Renders correctly on modern desktop and mobile browsers (Chrome, Firefox, Safari)
• Required fields prevent submission until valid, with no console errors
• Code passes W3C validation

If you can craft a tidy, flexible solution quickly, I’m ready to move forward.