We need to build a single-page web app (index.html) with CSS, JS inline. Should contain a 16-step sequencer grid for four instruments: kick, snare, hi-hat, clap. Sounds synthesized using Web Audio API (no external samples). Provide play/stop button and adjustable BPM (probably via slider or input). All in one file.

We'll create index.html in /workspace.

First, check if any existing files.

Let's start with bash ls.

We need to create index.html. We'll write the content.

Design: A container with title, BPM control, play/stop button, then a grid: rows for each instrument, columns for 16 steps. Each cell clickable to toggle active/inactive.

We'll synthesize sounds using OscillatorNode + envelopes etc. For kick: low frequency sine with pitch envelope. Snare: noise + sine. Hi-hat: noise with fast decay. Clap: multiple noise bursts or filtered noise.

Simplify: Use AudioContext, create sound functions that return AudioBufferSourceNode or directly play.

We'll implement a sequencer that steps through 16th notes based on BPM.

We'll store pattern as 4x16 boolean matrix.

We'll need to handle user interaction: clicking cells toggles.

We'll implement play loop using setTimeout or better using Web Audio API's schedule with AudioContext.currentTime and a timer using setInterval for simplicity.

We'll adjust BPM via input range (e.g., 60-200). Display current BPM.

All in one file.

Let's craft the HTML.

We'll write to file.

We'll first create a temporary content variable then write.

Let's use write tool.

We tried to write to /index.html but the filesystem is read-only? Actually earlier we saw we are in /workspace and it's writable. Let's try to write to ./index.html (current directory). Use path "index.html".

Now verify file exists and maybe test quickly. Let's read first few lines to ensure content.

We have created the file. Should we test? Not necessary, but we can run a quick check that the file is present and maybe open in a browser? Not needed. The task is to build the single-page web app and write it into index.html. Done.

We'll finalize.