Okay, I need to create a playable Snake game in a single HTML file. Let me think about how to structure this.

First, the HTML structure. I'll need a canvas element where the game will be rendered. Also, I should include some basic styling in a <style> tag. The JavaScript will be in a <script> tag. Since it's all self-contained, no external files.

For the canvas, I'll set a width and height. Maybe 400x400 pixels. The snake will move in a grid, so each segment is a certain size, like 20x20 pixels. That way, the grid is 20x20 cells.

Next, the JavaScript. I'll need variables for the snake, food, direction, score, and game state. The snake can be an array of segments, each with x and y coordinates. The food will have random x and y positions.

The game loop will use requestAnimationFrame to keep it smooth. The snake moves by adding a new head in the current direction and removing the tail unless it's eating food. When food is eaten, the tail doesn't shorten, so the snake grows.

Handling keyboard input: listen for keydown events and change the direction accordingly. But need to prevent reversing direction, like not allowing left if moving right.

Collision detection: check if the head hits the wall or itself. If so, trigger game over.

Displaying the score and game over message. When game over, show a message and a restart option, maybe by pressing a key.

Testing the code. Make sure the snake moves, grows, and the game resets properly. Also, check that the canvas is cleared and redrawn each frame.

Putting it all together in the HTML file. Make sure the <style> and <script> tags are correctly placed. No external resources, so all code is inline.

Wait, what about the initial setup? The canvas context needs to be obtained, and the initial snake and food positions set. Also, the game loop function that updates and draws everything.

I should also handle the game over state, stopping the loop and displaying the message. Restarting the game by resetting all variables.

Potential issues: The snake moving too fast. Maybe use a delay between movements, but with requestAnimationFrame, it's better to control the speed with a variable and a timer.

Another thing: when the snake eats food, generate new food in a random position that's not on the snake.

Testing the code step by step. Maybe start with the basic movement, then add the food, then the tail growth, then collision detection, and finally the game over and restart.

Okay, I think I have a plan. Now, write the code accordingly.

Okay, I need to check if the user's request for a Snake game was properly addressed. The assistant created a single HTML file with embedded CSS and JavaScript, which meets the requirement of being self-contained. The code includes a canvas for rendering, handles arrow key controls, manages the snake's movement and growth, tracks the score, and implements game over and restart functionality.

I should verify that all parts of the query are covered. The game has a growing tail (by not removing the last segment when food is eaten), food, score, game-over state, and restart. The code uses only inline resources, no external files. The HTML structure is correct with proper tags. The JavaScript is correctly placed within a script tag. The CSS is in a style tag. The file is named index.html as requested.

Potential issues: The initial direction might be undefined, but the code starts with direction {x:0, y:0}, which should prevent immediate movement. The game loop uses requestAnimationFrame, which is appropriate. Collision detection checks for wall and self-collision. The restart function resets all variables and starts the loop again. The food is regenerated correctly when eaten.

Looks like the solution is complete and meets all the user's specifications. The assistant did a good job. No further actions needed.