Advanced Tic-Tac-Toe Game

Build a self-contained HTML file (with optional external CSS and JavaScript) that runs an unbeatable Tic-Tac-Toe. The only gameplay requirement is a single-player mode where the user challenges a hard-level AI; the computer should never lose, so please rely on a full minimax (or equivalent) algorithm rather than a simple heuristic.

I haven’t settled on a visual style yet, so feel free to propose either a sleek minimal look or something more vibrant—clean, readable code and smooth user interactions matter more to me than specific colors. The board must be responsive and work equally well on desktop and mobile browsers without external dependencies other than standard web APIs.

Key deliverables:
• Complete HTML/CSS/JS source with clear comments
• Hard-difficulty AI that plays optimally
• Lightweight user interface with turn indicators and end-game messages (win, lose, draw)
• Brief README explaining file structure and how to tweak styles or AI depth

Acceptance criteria:
1. Opening the HTML file in any modern browser allows immediate play—no build step.
2. The AI never loses over 50 consecutive test games.
3. Code passes through W3C validator with no critical errors.

Once these points are met I can handle hosting and optional score tracking myself. Looking forward to seeing your efficient, well-structured solution.