We need to write a single html file that renders an animated synthwave sunset. Use CSS & JS. Probably use canvas for animation. Use CSS for background gradient, but need scanlines, grid, starfield, reflection. Use double canvas or light effect.

We want fully self-contained, no external libraries. We can use Canvas API for parallax. We can implement:

- Main canvas fill black.
- Draw background gradient (black to purple to pink to orange etc.) maybe animate slight shift.
- Draw starfield: small bright dots moving slowly.
- Draw grid: horizontal lines spaced, moving down (or up) to simulate scrolling.
- Draw sun: big circle with radial gradient, orange.
- Scanlines: maybe use narrow lines alternating transparent and semi-transparent using pattern overlay.

- Reflection: maybe render sun onto inverted canvas below horizon but fading.

Plan: create canvas that covers viewport. Use requestAnimationFrame for animation at ~60fps.

Implementation details:

- Use `window.requestAnimationFrame` to update.
- In each frame, clear canvas, draw background gradient (maybe not animated).
- Draw stars: array of stars with random x, y, speed. In update, y += speed; reset to top with random x if off bottom.
- Draw grid: create lines at multiple heights. Use offset variable to scroll. Keep offset increasing; when exceed grid spacing, wrap.

- Use `ctx.lineWidth`, `ctx.strokeStyle` for grid lines.

- For scanlines: create offscreen canvas with repeating horizontal lines 2-4px; set as pattern with globalAlpha 0.3; overlay by fillRect.

- Sun: just draw circle at screen center? but want it setting. perhaps animate sun's y position moving down until horizon. The reflection: draw sun again inverted with opacity decreasing.

- Use CSS for overall page minimal.

Ok.

We need to write index.html file. Use <style> and <script>.

Let's produce code.

Implementation detail: responsive: canvas width/height to window.innerWidth/Height.

10 stars, grid lines 8 px spacing maybe 10 lines at a time.

We also need amplitude: infinite grid scrolling towards viewer: gradient modifies? Instead of just static, we can animate color of grid lines from i think.

Ok.

Let's write.

We need to handle resource: All static.

Let's craft HTML skeleton.

We'll create a <canvas id="canvas"></canvas> that fills page.

CSS: body margin 0; overflow hidden; canvas position fixed 0 0.

Script: onload create canvas context, set canvas width/height.

Define data structures.

We can maybe create star array:

const stars = Array.from({length: 120}, () => ({
  x: Math.random()*w, y: Math.random()*h, radius: Math.random()*1.5, speed: 0.02 + Math.random()*0.04
}));

For gradient sun: r = 80, color radial gradient : 'rgba(255,100,0,1)' to 'rgba(255,200,0,0.2)'

Sun y coordinate: start -50 > bottom.. not needed, but we set y relative to 0.3h.

Set reflection: 40% brightness, at y = i (height - sunY + offset) maybe.

Add event listener for resize.

Use setTimeout 0 for canvas.

Let's produce code.

We need to ensure "self-contained" i.e., no external libs.

Ok.

Let's write the index.html. We'll also test quickly with OpenAI environment? We can't run.

But we shall produce correct HTML.

Make sure to use `/*` to comment and then `*/`. For readability.

Ok.

Let's produce final output.