Okay, I need to create a single-page Tetris game in one HTML file. Let's start by outlining the structure. The HTML will have a canvas element for the game and some elements for the next piece preview and score.

First, the canvas setup. I'll need to set the width and height, maybe 200x400 pixels. Then, the CSS for the body to center everything. The next piece preview can be another canvas or a div with colored blocks. The score display will be a simple text element.

For the JavaScript, the main components are the game loop, tetromino shapes, rotation logic, collision detection, line clearing, and scoring. Let's think about the tetrominoes. Each has a shape and color. The seven standard ones: I, O, T, S, Z, J, L. Each can be represented as a 2D array.

The game state will include the grid, the current piece, the next piece, the score, and the game speed. The grid is a 2D array where each cell is 0 (empty) or a color. The current piece has a position and shape. When moving down, check for collisions. If collision, lock the piece into the grid and check for lines to clear.

Rotation is a bit tricky. Each piece has different rotation states. Need to rotate the shape and check if it fits. If not, maybe adjust the position (like in some games, the piece shifts to the side if rotation would cause a collision).

Line clearing: check each row from bottom up. If a row is full, remove it and shift the rows above down. Add to the score based on the number of lines cleared.

The game loop uses setInterval to move the piece down at increasing intervals as the score increases. Also, handle user input for left, right, down, and rotation.

For the next piece preview, display the next tetromino's shape in a smaller grid. The score updates as lines are cleared.

Now, putting all this into the HTML. The canvas for the game, another canvas or div for the next piece, and a div for the score. The JavaScript will be inline in a script tag.

Testing for possible errors: ensuring that the collision detection works, rotation handles all cases, line clearing is efficient, and the game speed increases properly. Also, making sure that the game ends when the pieces stack to the top.

I'll start writing the HTML structure, then the CSS, then the JavaScript. Make sure all code is self-contained, no external resources. Use the canvas API for drawing the game and the preview.

Okay, the user asked for a playable Tetris game in a single HTML file. I need to make sure all the components are included: the canvas, CSS styling, JavaScript for game logic, and everything self-contained.

First, I'll check the structure. The HTML has a canvas for the game, another for the next piece, and a score display. The CSS centers everything and styles the elements. The JavaScript handles the game loop, piece movement, rotation, collision detection, line clearing, and scoring.

I should verify that all tetromino shapes and colors are correctly defined. The SHAPES and COLORS objects look right, covering all seven pieces. The grid is initialized properly with createGrid(), and the getRandomPiece() function selects random pieces.

The draw functions (drawGrid, drawPiece, drawNextPiece) use the canvas API to render the game state. Collision detection with collide() checks for boundaries and existing blocks. Merging the piece into the grid when it can't move down anymore is handled by merge().

Line clearing in clearLines() checks each row and removes full lines, updating the score. The rotation function rotates the matrix and checks if it's valid. The game loop uses setInterval to control the speed, which increases as the score goes up.

Event listeners for arrow keys handle movement and rotation. The game ends when a new piece can't be placed, triggering an alert and reload.

I think that covers everything. The code is all in one file, no external resources, and should work when opened in a browser. Testing might be needed for edge cases, but the core mechanics are there.