Build a 2048 game in a single index.html file. Standard implementation: 4x4 grid, arrow keys, merging, score, win overlay at 2048, game over detection, smooth animations using absolutely positioned tiles with transitions.

I'll write it carefully. Animation approach: maintain a board as a grid of tiles with unique ids, render tiles as absolutely positioned divs, use CSS transitions on transform/position. On move, compute the new positions and merges, animate, then finalize.

A simpler, more robust approach: track tiles as an array of {id, value, row, col}. On move, process each line, compute the post-move positions and merges. Update the DOM: move existing tile elements, and when merging, slide both tiles to the target position, then remove one and bump the value with a "pop" animation.

Let's write the code.

Key logic:
- state: 4x4 grid of tile objects or null.
- move(dir): traverse rows/columns in order; for each tile, find the farthest empty cell and the next cell; if the next cell has the same value and hasn't yet merged this turn, merge.
- Track moved flag; after the move, spawn a random tile (2 or 4) and check win/loss.

Rendering: absolutely position tiles inside the board container; use CSS transitions on transform. After a merge, render a merged tile with a pop animation.

Let's write the file.

Verify quickly with node that there are no syntax errors? Extract the script and check it.

Maybe I should also run a simple logic simulation of the merge algorithm, but it's probably fine. One thing to double-check: the merge animation — on merge, the moving tile slides to the target position (transform transition 130ms), then after a 130ms timeout, we delete the moving tile and update the target tile's value with a pop animation. The pop animation includes scale(0) at 0%, so the tile momentarily disappears — this is the standard 2048-style behavior; actually, the original starts the pop from scale(0)→1.2→1. No problem.

Also verifying: does `won && !keepPlaying` trigger the overlay on every move afterward? After the win overlay is shown, `move` is blocked by the `won && !keepPlaying` guard. If you continue, keepPlaying becomes true. No problem.

Game over check: after each move, if there are no empty cells and no merges, over=true. No problem.

Done.