The user wants a 16-step drum machine as a single `index.html` file.
Requirements:
- 16 steps.
- 4 sounds: Kick, Snare, Hi-hat, Clap.
- Synthesized using Web Audio API (no external samples).
- Clickable grid for triggering.
- Play/Stop controls.
- Adjustable BPM.
- Self-contained (CSS in `<style>`, JS in `<script>`, no external deps).

Plan:
1.  Define the HTML structure: a grid for the steps, controls for BPM and Play/Stop.
2.  Implement the CSS for a clean, dark-themed "studio" look.
3.  Implement the Web Audio API synthesis for each sound:
    - Kick: Sine wave with rapid pitch drop.
    - Snare: White noise with a fast decay and a low-frequency thump.
    - Hi-hat: High-pass filtered white noise with very short decay.
    - Clap: Short bursts of filtered noise with slight randomization.
4.  Implement the sequencer logic:
    - A `requestAnimationFrame` or `setInterval` based loop (better to use the Web Audio API clock for precision).
    - Tracking the current step.
    - Triggering sounds based on the grid state.
5.  Handle user interactions: clicking the grid to toggle steps, adjusting BPM, and toggling playback.

Let's refine the sound synthesis:
- Kick: oscillator (sine), freq start ~150Hz -> 40Hz over 0.1s.
- Snare: noise buffer + oscillator (sine ~200Hz).
- Hi-hat: noise buffer + high-pass filter.
- Clap: noise buffer with a specific envelope (multiple short peaks).

Wait, creating a noise buffer is standard. I'll generate it once and reuse it.

Let's start writing the code.

The user wants a 16-step drum machine. I have already provided a complete `index.html` with:
1. 16-step grid for Kick, Snare, Hi-hat, and Clap.
2. Web Audio API synthesis for all four sounds (avoiding external samples).
3. Play/Stop functionality.
4. Adjustable BPM (60-200).
5. Single-page, self-contained CSS/JS.
6. No external resources/CDNs.

I should double-check if I missed anything.
- Clickable grid: Yes.
- Synthesized sounds: Yes (Oscillators and noise buffers).
- Play/Stop: Yes.
- Adjustable BPM: Yes.
- All in one file: Yes.

The code looks solid. The `triggerSound` function handles the scheduling using `AUDIO.currentTime` and `time` for precise timing, which is a best practice for Web Audio apps to avoid jitter.

I'll do a final check on the `clap` synthesis. Claps are tricky. I used a sparse noise burst, which is a basic way to simulate it. The snare uses noise + triangle thump. The kick uses a pitch-dropping oscillator. Hi-hat uses high-pass filtered noise. These are standard additive/subtractive synthesis techniques for drum machines.

One detail: `nextNoteTime += 0.25 * secondsPerBeat;`. 
If BPM is 120, `secondsPerBeat` is 0.5. `0.25 * 0.5 = 0.125`.
16 steps at 0.125s per step = 2 seconds per loop.
120 BPM / 60 = 2 beats per second. 
2 beats per second * 4 steps per beat = 8 steps per second.
16 steps / 8 steps per second = 2 seconds.
Correct.

Final verification of requirements:
- 16-step: Yes.
- Clickable grid: Yes.
- Kick, snare, hi-hat, clap: Yes.
- Synthesized with Web Audio API: Yes.
- Play/stop: Yes.
- Adjustable BPM: Yes.
- Self-contained index.html: Yes.
- No external resources: Yes.

The solution is complete.