The user wants a 16-step drum machine as a single-page web app (`index.html`).
Key requirements:
- 16 steps.
- 4 sounds: kick, snare, hi-hat, clap.
- Synthesized sounds using Web Audio API (no external samples).
- Clickable grid for sequencing.
- Play/Stop buttons.
- Adjustable BPM.
- Single file: CSS in `<style>`, JS in `<script>`.
- No external resources/CDNs.

Plan:
1. HTML structure:
    - Title.
    - Controls: BPM input, Play/Stop button.
    - Grid: 4 rows (one for each instrument) x 16 columns (steps).
2. CSS:
    - Basic styling for the grid (cells), controls, and layout.
    - Active/inactive states for cells.
    - Highlighting the current step during playback.
3. JavaScript:
    - Web Audio API setup (AudioContext).
    - Synthesizer functions for Kick, Snare, Hi-hat, and Clap.
    - Sequencing logic:
        - State: Current step, BPM, isPlaying, grid data.
        - `scheduler` or `setInterval`/`requestAnimationFrame` for timing (though `AudioContext.currentTime` is better for precision).
        - A loop that triggers sounds based on the grid state.
    - Event listeners for clicking cells and controlling playback.

Let's design the synth sounds:
- Kick: Sine wave sweep from high to low frequency + short decay.
- Snare: White noise with a bandpass filter + short decay, and maybe a low-frequency "pop" sine wave.
- Hi-hat: High-pass filtered white noise with very short decay.
- Clap: A burst of noise with a specific envelope (multiple short peaks) or just a filtered noise burst.

Implementation details for Timing:
To avoid jitter, I'll use a "look-ahead" scheduler.
- `nextNoteTime` keeps track of when the next step should occur.
- A `scheduler()` function runs frequently and schedules notes for the next few hundred milliseconds using `oscillator.start(time)`.

Let's write the code.

The user wants a 16-step drum machine.
Key requirements:
- Single file `index.html`.
- No external resources (CDNs, network requests).
- 16-step grid for: Kick, Snare, Hi-hat, Clap.
- Sounds synthesized using Web Audio API (no audio files).
- Play/Stop functionality.
- Adjustable BPM.
- CSS in `<style>`, JS in `<script>`.

Plan:
1.  HTML Structure:
    - Header with title.
    - Controls: Play/Stop button, BPM input.
    - Grid: 4 rows (instruments) x 16 columns (steps).
2.  CSS Styling:
    - Modern dark theme.
    - Grid layout for steps.
    - Visual feedback for the current step (playhead).
    - Active state for toggled steps.
3.  JS Logic:
    - Web Audio API context initialization.
    - Sound synthesis functions for:
        - Kick: Frequency sweep (sine wave).
        - Snare: White noise + short burst of low frequency.
        - Hi-hat: High-pass filtered white noise.
        - Clap: Short bursts of noise with a specific envelope.
    - Sequencer loop:
        - Use `setInterval` or `requestAnimationFrame` but preferably a precise Web Audio clock for timing.
        - Track current step (0-15).
        - Trigger corresponding sounds if the step is active.
    - Event handlers for clicking the grid and updating BPM.

Let's refine the sound synthesis:
- Kick: Oscillator (sine), fast pitch drop from ~150Hz to ~40Hz.
- Snare: White noise with a short decay, combined with a triangle wave (~180Hz) for the "snap".
- Hi-hat: White noise with a high-pass filter and very short decay.
- Clap: Several short bursts of noise closely spaced to simulate the hand-clap "cluster" effect.

Let's implement.