The user wants a single-page web app with a Space Invaders game built using HTML5 Canvas. They want:

1. A player cannon that moves and shoots
2. A descending grid of aliens that advance and fire back
3. Destructible bunkers
4. Score and lives tracking
5. Procedural graphics only (no external assets)
6. Everything in one self-contained index.html file
7. No external resources, CDNs, or network requests

I need to create a complete, functional Space Invaders game using only HTML, CSS (in a style tag), and JavaScript (in a script tag). Everything should be self-contained in a single index.html file.

Let me plan the structure:

1. HTML structure with canvas element
2. CSS for styling (including game over screen, UI)
3. JavaScript with game logic:
   - Game state management
   - Player control (movement and shooting)
   - Alien behavior (movement pattern, firing)
   - Bullet management (player and alien bullets)
   - Bunker system (destructible barriers)
   - Collision detection
   - Scoring and lives
   - Animation loop
   - Procedural graphics (drawing everything using canvas)

Let me start by creating the index.html file with the basic structure.