The user wants me to build a complete Tetris game in a single HTML file with a canvas element. Let me plan out all the features:

1. All 7 tetrominoes (I, O, T, S, Z, J, L)
2. Rotation (with wall kicks)
3. Line clears
4. Increasing speed
5. Next piece preview
6. Score display
7. Canvas-based rendering

Let me build this comprehensively.

Let me verify the file was written correctly and check for any issues.

The file is 453 lines and looks well-formed. Let me double-check the game logic once more quickly by reviewing the key parts to make sure everything is correct.

Actually, let me also check if there's a subtle issue: when the game starts, `resetGame()` calls `spawnNext()`, which sets `current` and `next`. But `spawnNext()` checks `if (!next)` - on the first call, `next` is null, so it assigns `next = randomPiece()`. Then it sets `current = spawnPiece(next)` and `next = randomPiece()`. That's correct.

Wait, there's a bug in `spawnNext()`: the first time it's called, `next` is null, so it sets `next = randomPiece()`. Then `current = spawnPiece(next)` and `next = randomPiece()`. That's fine - current gets a piece, next gets a new random piece.

But wait, let me re-read:
```
function spawnNext() {
    if (!next) {
      next = randomPiece();
    }
    current = spawnPiece(next);
    next = randomPiece();
    ...
}
```

First call: next is null, so next = randomPiece(). Then current = spawnPiece(next). Then next = randomPiece(). Good.

Second call: next is not null (it was set in the previous call). So we skip the if. current = spawnPiece(next). Then next = randomPiece(). Good.

But actually, wait. In `resetGame()`, we call `spawnNext()`. But we also set `next = null` before that. So on the first call:
- next is null, so next = randomPiece() (first piece)
- current = spawnPiece(next) (current gets the piece)
- next = randomPiece() (next gets a new piece)

That's correct.

In `lockPiece()`, we call `spawnNext()` again. At that point, next has a piece from the previous call. So:
- next is not null, skip the if
- current = spawnPiece(next) (current gets the waiting piece)
- next = randomPiece() (next gets a new piece)

That's correct! The bag randomizer works well.

One thing I notice: I should also check that the animation loop keeps running. The `render()` function calls `requestAnimationFrame(render)` at the end, so it will keep running once started. The `resetGame()` function calls `render()` only if `animationId` is falsy, which it will be on the first call. After that, the loop continues.

Let me also think about the `valid` function for the ghost piece - when the current piece is at the top and some rows are above the board (y < 0), the `isValid` function checks `if (by < 0) continue;` which is fine for collision detection.

The game looks correct. Let me also verify the file is complete by reading it.

Actually, I think the file is good. The game has all the requested features:
- All 7 tetrominoes ✓
- Rotation (with wall kicks for I piece and others) ✓
- Line clears ✓
- Increasing speed (level up every 10 lines, drop interval decreases) ✓
- Next-piece preview ✓
- Score (100/300/500/800 points for 1-4 lines, multiplied by level, +2 per hard drop row) ✓
- Ghost piece ✓
- Bag randomizer for fair piece distribution ✓
- Game over detection ✓
- Restart button ✓

I'm satisfied with the implementation.