The user wants me to create a single-page web app that implements a playable piano keyboard. Let me break down the requirements:

1. Use the pre-provided `tone.min.js` (global Tone)
2. Build a playable piano keyboard controlled by mouse and computer keys
3. Synthesizes notes with an ADSR envelope
4. Visual key feedback
5. All in one self-contained `index.html` file with CSS in `<style>` and JS in `<script>` tags
6. No external resources, CDNs, or network requests
7. Load `tone.min.js` with a plain `<script src="tone.min.js"></script>`

I need to create a complete piano implementation that:
- Has a visual keyboard (likely standard octave layout)
- Can be played with mouse clicks
- Can be played with keyboard keys (mapping computer keyboard to piano keys)
- Uses Tone.js for sound synthesis with ADSR envelope
- Shows visual feedback when keys are pressed

Let me think about the implementation:

1. HTML structure:
   - A container for the piano keys
   - Keys arranged in a row, white keys and black keys positioned appropriately
   - Each key should be a div or button element

2. CSS:
   - Style the piano keys to look like a keyboard
   - Position black keys above and between white keys
   - Add visual feedback states (pressed/active)

3. JavaScript:
   - Load Tone.js (already provided)
   - Map keyboard keys to piano notes (e.g., A->C, S->D, etc.)
   - Handle mouse clicks on keys
   - Handle keyboard events (keydown, keyup)
   - Create Tone.js synthesizer with ADSR envelope
   - Play notes when triggered
   - Add visual feedback (add/remove CSS classes)

I'll need to ensure the piano covers a decent range, maybe 1-2 octaves. Let's plan for one full octave (C4 to C5) or maybe a bit more.

Key mappings (common piano mapping):
   - White keys: a s d f g h j k l ;
   - Black keys: w e y u o p
   - Could also include z x c v b n m for lower octave

But for simplicity, I'll do a single octave: C4, C#4, D4, D#4, E4, F4, F#4, G4, G#4, A4, A#4, B4, C5

Standard mapping:
- C: a
- C#: w
- D: s
- D#: e
- E: d
- F: f
- F#: t
- G: g
- G#: y
- A: h
- A#: u
- B: j
- C5: k

But I need to ensure it feels natural. Let me check typical piano keyboard apps:
Typically:
Row: Z X C V B N M , . / for lower octave
Row above: Q W E R T Y U I O P for upper notes
Or sometimes: A S D F G H J K L ; for white keys and W E Y U O P for black keys

I'll go with the common:
White: a s d f g h j k l ;
Black: w e y u o p

But I'll make it configurable via data attributes.

Let's code the full solution:

- index.html will have:
  - `<script src="tone.min.js"></script>`
  - `<style>` for piano styling
  - `<div id="piano">` with keys
  - `<script>` for logic

I need to be careful with the timing of loading Tone.js and starting audio. Tone.js requires user interaction to start audio context. I'll need to initialize the synth on first user interaction.

Let me create a complete implementation.

Also, for the ADSR envelope, I'll configure a Tone.Synth or Tone.MembraneSynth/AMSynth with appropriate ADSR values.

Actually, for a piano sound, a sampled piano would be ideal, but we're using a synth. I can use Tone.Synth with a "piano-ish" envelope or use Tone.FMSynth for a more metallic sound. But a basic PolySynth with a piano-like envelope (fast attack, medium decay) could work.

Better: Use Tone.PolySynth with a custom envelope or multiple oscillators.

Piano characteristics:
- Fast attack (very quick)
- Medium decay
- No sustain or minimal sustain
- Release after key release

I'll use:
```javascript
const synth = new Tone.PolySynth(Tone.Synth, {
  envelope: {
    attack: 0.005,
    decay: 0.5,
    sustain: 0,
    release: 1
  },
  oscillator: {
    type: "triangle" // or "sine" for softer
  }
}).toDestination();
```

Or better, use a more complex synth like MembraneSynth for a percussive sound.

Alternatively, I could use a Sampler but we don't have samples.

Let's stick with a PolySynth with triangle waves and an ADSR envelope.

Now, the keyboard layout. I'll create a piano with one octave + one extra C (13 keys). Each key will be a div with a data-note attribute for the note (like "C4") and a data-key attribute for the keyboard key.

The HTML structure:
```html
<div id="piano">
  <div class="key white" data-note="C4" data-key="a">C</div>
  <div class="key black" data-note="C#4" data-key="w">C#</div>
  <div class="key white" data-note="D4" data-key="s">D</div>
  ...
</div>
```

CSS:
- The piano container: display: flex; position: relative;
- White keys: flex: 1; order matters; positioned in a row
- Black keys: absolute positioning between white keys, or use negative margins and z-index

Simpler: Use a container for white keys, and black keys positioned absolutely relative to container, with left percentages.

Common technique: Have all keys in a single flex container, but assign white keys a width and black keys narrower, positioned with margin-left: -halfWidth etc.

But easier: Use two rows? Not really.

I'll use a single container with `display: flex` for white keys, and black keys absolutely positioned.

Actually, a simpler approach: Each key is a div. The container sets the positions. Use CSS grid? Not needed.

Standard approach: Container has `position: relative`. White keys are flex items or block with floats. Black keys are absolutely positioned with left calculated based on the position of the previous white key.

But for a standalone demo with a fixed number of keys, I can just hardcode positions using CSS.

Better: Use flex for white keys and position black keys with `left: X%` based on the index.

Let me calculate: For 7 white keys per octave, each white key is about 1/7 of width. Black keys are centered on the line between white keys except after E and B where there is no black key.

I'll create 7 white keys (C, D, E, F, G, A, B, C) actually 8? Wait, one octave from C to B is 7 white keys, plus next C is 8. So I'll have 8 white keys for two octaves? Let's stick to one octave plus the C above: C4, D4, E4, F4, G4, A4, B4, C5 -> that's 8 white keys. And black keys: C#4, D#4, F#4, G#4, A#4 -> that's 5 black keys. Actually also maybe F#4? Yes. So total 13 keys.

But I'll just implement the C major scale plus sharps.

To simplify CSS: I'll use a flex container for white keys only, and then black keys as children of the container but positioned absolutely with `left: X%`. I'll assign each black key a specific CSS class like `black-1`, `black-2` etc.

But to keep HTML clean, I'll compute positions inline via style attribute? Could do.

Simplify: Use a table-like structure? Not ideal.

Alternate: Use CSS Grid: define columns for each white key, and black keys placed in the grid cells with negative margins? Actually grid could work: define 8 columns for white keys, and black keys placed spanning between columns.

But for simplicity and to avoid complex CSS, I'll just use absolute positioning and calculate left positions in percentages. In the HTML, each key will have a class and maybe a data-position.

Let's enumerate the white keys (positions from left):
1. C4 (index 0)
2. D4 (1)
3. E4 (2)
4. F4 (3)
5. G4 (4)
6. A4 (5)
7. B4 (6)
8. C5 (7)

Black keys:
- C#4: between C4 and D4, roughly at 1/14 (or 0.5 * whiteKeyWidth offset from C4)
- D#4: between D4 and E4: position 1 + 0.5 whiteKeyWidth offset = 1.5 * whiteKeyWidth from left
- F#4: between F4 and G4: position 3 + 0.5 = 3.5
- G#4: between G4 and A4: 4.5
- A#4: between A4 and B5: 5.5

Since we have 8 white keys, each white key width = 100/8 = 12.5% (if container is 100%). But we want the black keys to be narrower (maybe 8% width) and centered on the boundary.

Actually, typical piano: white keys are 1 unit, black keys are ~0.6 unit width and centered on the line between white keys (offset by half a white key minus half black width). So if white key width = w, black key left = (i + 0.5) * w - (blackWidth/2).

But if we set black key width as maybe 8% and white key width 12.5%, then the left offset = (i * 12.5%) + (12.5%/2) - (8%/2) = i*12.5% + 6.25% - 4% = i*12.5% + 2.25%.

So positions:
- C# (after 1st white): i=0 => left: 2.25%
- D# (after 2nd): i=1 => left: 12.5% + 2.25% = 14.75%
- F#: after F (i=3) => 3*12.5% + 2.25% = 37.5% + 2.25% = 39.75%
- G#: after G (i=4) => 50% + 2.25% = 52.25%
- A#: after A (i=5) => 62.5% + 2.25% = 64.75%

But there is no black after E (i=2) and after B (i=6) because those are the natural half steps.

This seems doable. However, this gets a bit messy. Maybe simpler: Use a flexbox approach with white keys as flex items and black keys as positioned absolute relative to the container, using left based on the index of the white key they follow. That is what I described.

Alternatively, I can use a more pure CSS approach: put both white and black keys in a single container, set white keys to have `position: relative` and black keys to have `position: absolute` with left set to a specific pixel or percentage. Since the container width is fixed (like 100vw or a specific pixel), percentages work.

But to avoid complex calculations, maybe I'll use CSS variables and set the number of keys, but that's overkill.

I'll go with the flex container approach:

```html
<div class="piano">
  <!-- White keys first -->
  <div class="key white" data-note="C4" data-key="a"></div>
  <div class="key white" data-note="D4" data-key="s"></div>
  <div class="key white" data-note="E4" data-key="d"></div>
  <div class="key white" data-note="F4" data-key="f"></div>
  <div class="key white" data-note="G4" data-key="g"></div>
  <div class="key white" data-note="A4" data-key="h"></div>
  <div class="key white" data-note="B4" data-key="j"></div>
  <div class="key white" data-note="C5" data-key="k"></div>
  <!-- Black keys as absolute positioned -->
  <div class="key black" data-note="C#4" data-key="w" style="left: 7%;"></div>
  <div class="key black" data-note="D#4" data-key="e" style="left: 18.5%;"></div>
  <div class="key black" data-note="F#4" data-key="t" style="left: 45%;"></div>
  <div class="key black" data-note="G#4" data-key="y" style="left: 57.5%;"></div>
  <div class="key black" data-note="A#4" data-key="u" style="left: 70%;"></div>
</div>
```

But I'd like to compute accurate percentages. Let's determine the width of the piano container. Suppose we set container width to something like 800px. Then white keys each ~100px. Black keys might be 60px wide, positioned 50px from the left of the white key's start? Actually easier: Use percentages as earlier.

But to make it look right, I'll use a more robust method: set white keys to `flex: 1` so they equally divide container width. The container will have `display: flex`. Then the black keys can be positioned with `left: calc( (whiteKeyIndex) * (100% / numWhiteKeys) + (100% / numWhiteKeys / 2) - (blackKeyWidth/2) )`. But we need to define a black key width in %. I can set a CSS variable for black key width and compute with `calc`. However, we can't easily compute arbitrary percentages in pure CSS without pre-calculating. For simplicity, I'll pre-calculate using 8 white keys, each 12.5% width. For black key width, say 8% of container. Then the formula above: left = i * 12.5% + 6.25% - 4% = i*12.5% + 2.25%. For i values:
- C#: i=0 -> 2.25%
- D#: i=1 -> 14.75%
- F#: i=3 -> 39.75%
- G#: i=4 -> 52.25%
- A#: i=5 -> 64.75%

But we also need to ensure the black key sits above the boundary, not overlapping incorrectly. The black key should be centered between two white keys. That seems right.

I'll also add extra keys for maybe a lower octave? But I'll start with this and ensure it works.

Also, need to consider that black keys are above white keys, so they have higher z-index and negative margin? Actually they will be absolutely positioned relative to the container, so they will be placed on top (we can set z-index).

The container `.piano` will have `position: relative`. The white keys will be `display: inline-block` or flex items with `position: relative`? Actually white keys can be normal flow, and black keys absolute with top: 0 (or slightly offset). We need to set white keys to have height, and black keys to be shorter and positioned at the top. Usually black keys are shorter and sit above the white keys.

I'll set:
- .key: common styles
- .white: background white, border, height: 200px, width: 12.5% (since 8 keys) but using flex they will auto fill.
- .black: background black, height: 120px, width: 8% (pre-calc), position: absolute, top: 0, left: as above, z-index: 2.

But if I set white keys as flex items in a flex container, the container's width is the full width. The black keys absolute positioning left in % is relative to container, which works.

I need to ensure that the container's height is determined by the white keys (taller). The black keys are absolute, so they won't affect height. That's fine.

Now, the interaction:

1. On mousedown on a key:
   - Prevent default
   - Trigger note on (synth.keyDown(note))
   - Add visual active class

2. On mouseup or mouseleave:
   - synth.keyUp(note)
   - Remove active class

For keyboard events:
   - On keydown: find element with matching data-key, if not already playing, trigger noteOn and add active class
   - On keyup: find matching element, trigger noteOff and remove active class

But need to handle key repeat: keydown fires repeatedly while key held. We want to trigger noteOn only once per key press. Use a Set to track currently pressed keys.

Implementation details:

- Wait for user interaction to start audio context. Tone.js is blocked until a user gesture. We can do `await Tone.start()` on the first interaction (like any mousedown or keydown). Or we can add an overlay to click to start. Simpler: on the first key press (mouse or keyboard), call `Tone.start()` if not already started.

- Use Tone.PolySynth to allow polyphony (multiple keys simultaneously). But the PolySynth already handles multiple notes. We can also use a single Synth if we want monophonic, but piano is polyphonic. So use PolySynth.

But note: Tone.PolySynth expects an array of notes. For keyDown we can call `synth.triggerAttack(note)`, and for keyUp `synth.triggerRelease(note)`. Or use `synth.keyDown()`? Actually PolySynth inherits from Monophonic, but the method is `triggerAttack` and `triggerRelease`. Or `triggerAttackRelease` for a note with duration. For ADSR, we want to control attack and release separately: on keydown, triggerAttack, on keyup triggerRelease.

I'll do:

```javascript
const synth = new Tone.PolySynth(Tone.Synth, {
  envelope: {
    attack: 0.005,
    decay: 1.5,
    sustain: 0,
    release: 2
  },
  oscillator: { type: "triangle" }
}).toDestination();
```

Then on note start: `synth.triggerAttack(note)`. On note end: `synth.triggerRelease(note)`.

Also need to set volume, maybe master gain.

Additionally, to emulate a piano's dampening, we could set a release time.

Now, the note frequencies: Tone.js can convert note names to frequency. We just pass note like "C4", "C#4", etc.

Mapping keyboard keys to notes:

I'll use:
- a: C4
- w: C#4
- s: D4
- e: D#4
- d: E4
- f: F4
- t: F#4
- g: G4
- y: G#4
- h: A4
- u: A#4
- j: B4
- k: C5

Optionally also include lower octave: z -> C3, etc. But I'll keep it simple for now.

But the piano has 13 keys (including black keys). I'll include all above. That's 13 mappings.

Now, visual feedback: add a CSS class `.active` to the key when pressed, which could change background color to something like #ccc for white keys (pressed) and #555 for black keys.

Also, maybe add a key label (like the letter) so user knows what to press.

Implementation plan:

1. Create HTML skeleton.
2. Add CSS for layout and styling.
3. Add JS to initialize Tone, set up event listeners, handle key presses.

I'll also add a volume control? Not necessary but could be nice. But requirement is just synthesizes notes with ADSR and visual feedback. So no need for extra controls.

Also, I'll ensure that the oscillator and envelope are configured properly.

Now, create the file.

First, let me check if tone.min.js exists in current directory. The user said "These library file(s) are ALREADY present in the current directory — load each with a plain <script src="NAME"></script> using its exact filename (never a CDN or URL)". So I should include `<script src="tone.min.js"></script>` in the HTML.

Now, build `index.html` with all code.

Let's write the code step by step.

HTML:

```html
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Piano</title>
  <style>
    /* CSS here */
  </style>
</head>
<body>
  <div class="container">
    <h1>Piano</h1>
    <div class="piano">
      <!-- white keys -->
      <div class="key white" data-note="C4" data-key="a">C</div>
      <div class="key white" data-note="D4" data-key="s">D</div>
      <div class="key white" data-note="E4" data-key="d">E</div>
      <div class="key white" data-note="F4" data-key="f">F</div>
      <div class="key white" data-note="G4" data-key="g">G</div>
      <div class="key white" data-note="A4" data-key="h">A</div>
      <div class="key white" data-note="B4" data-key="j">B</div>
      <div class="key white" data-note="C5" data-key="k">C</div>
      <!-- black keys -->
      <div class="key black" data-note="C#4" data-key="w">C#</div>
      <div class="key black" data-note="D#4" data-key="e">D#</div>
      <div class="key black" data-note="F#4" data-key="t">F#</div>
      <div class="key black" data-note="G#4" data-key="y">G#</div>
      <div class="key black" data-note="A#4" data-key="u">A#</div>
    </div>
    <p class="instructions">Use your mouse or keyboard (a-k for white keys, w,e,t,y,u for black keys)</p>
  </div>
  <script src="tone.min.js"></script>
  <script>
    // JavaScript here
  </script>
</body>
</html>
```

Now, CSS:

We need to style the piano keys. I'll aim for a reasonable look:

- Container: center piano, maybe some margin.
- .piano: position relative, display flex, width: maybe 800px or 100% max-width, height: 200px.
- .key: generic styles: cursor pointer, border, border-radius maybe, display flex, align-items: flex-end, justify-content: center, padding-bottom, user-select none.
- .white: background white, border: 1px solid #ccc, z-index: 1, height: 200px, flex: 1 (so they evenly fill the container). But if I use flex, each white key gets equal width. The black keys will be positioned absolute top: 0. They will not be part of flex flow.
- .black: background black, color white, width: 60px (or maybe percentage). But we need to position them precisely. I'll use percentages as calculated: left: 2.25%, 14.75%, 39.75%, 52.25%, 64.75%. And width: 6%? Actually, I need a consistent width. Let's decide: if white keys take 12.5% each (since 8 keys), then black key width should be about half of white key width minus a little. White key width = 12.5%. Black key width maybe 5% or 6%? Let's compute: white key = 12.5% of container width. Half white key = 6.25%. Black key width maybe 4%? Then the left offset formula: left = i*12.5% + (12.5%/2) - (4%/2) = i*12.5% + 6.25% - 2% = i*12.5% + 4.25%. That gives:
- i=0: 4.25%
- i=1: 16.75%
- i=3: 42.75%
- i=4: 55.25%
- i=5: 67.75%

I'll use 4% black width and compute left positions using that formula. That seems good.

But if I use percentages, they might not be pixel-perfect but should be fine.

Alternatively, to avoid complex percentages, I could set the container width to a fixed pixel value (like 800px) and calculate pixel positions. That might be simpler. I can set container width: 800px, then each white key = 100px (8 keys). Black key width = 60px. Then left positions:
- after C (0): (0*100) + (100/2) - (60/2) = 0 + 50 - 30 = 20px -> left: 20px
- after D (1): 100 + 50 - 30 = 120px
- after F (3): 300 + 50 - 30 = 320px
- after G (4): 400 + 50 - 30 = 420px
- after A (5): 500 + 50 - 30 = 520px

But also black keys need to be positioned relative to the container left. Since white keys are flex items starting at left: 0. So if container is 800px, white keys each 100px, but we need to be careful: flex items will fill the container exactly. So we can set container width: 800px; white key width: auto (flex:1). That means they will be exactly 100px each if there are 8 and container is 800px. So we can use that.

But then what about responsive? We could use percentages but then black key positions become percentages as well. Either way works. For simplicity and visual consistency, I'll use fixed pixel width. The piano will be centered and have a fixed width. That's okay.

So CSS:

```css
body {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  margin: 0;
  background: #222;
  font-family: sans-serif;
  color: #fff;
}
.container {
  text-align: center;
}
.piano {
  position: relative;
  width: 800px;
  height: 200px;
  background: #111;
  border-radius: 8px;
  padding: 10px;
  box-sizing: border-box;
  margin-top: 20px;
}
.key {
  position: relative;
  border: 1px solid #000;
  border-radius: 0 0 4px 4px;
  cursor: pointer;
  display: flex;
  align-items: flex-end;
  justify-content: center;
  padding-bottom: 10px;
  font-weight: bold;
  user-select: none;
  transition: background 0.1s;
}
.white {
  background: white;
  color: black;
  height: 100%;
  z-index: 1;
}
.black {
  background: black;
  color: white;
  width: 60px;
  height: 60%;
  top: 0;
  z-index: 2;
  /* left positions will be inline style */
}
.key.active {
  background: #ccc !important;
  color: #000 !important;
}
.key.black.active {
  background: #555 !important;
}
.instructions {
  margin-top: 10px;
  color: #aaa;
}
```

But with white keys as flex items, I need to ensure they fill the width equally. In the .piano container, .key.white will be direct children, and .key.black are also children but they have absolute positioning and won't take up flex space. So the flex layout will only consider white keys. That's fine.

But we need to ensure that the white keys are the only flex items and the black keys are absolute. However, the black keys are also children of .piano, but because they are absolute, they are removed from the normal flow. So the flex container will only lay out the white keys. That works.

Now, we have 8 white keys, so each will have equal width if we set .white { flex: 1; } or we can set .piano { display: flex; } and .white { flex: 1; }. Yes.

Now, the black keys need to be positioned exactly between the white keys. We'll use inline styles for each black key:

- C#4: left: 20px; (actually we need to compute: after first white key, left = (index of preceding white)*whiteKeyWidth + whiteKeyWidth/2 - blackWidth/2. Since whiteKeyWidth = container width / number of white keys = 800/8 = 100px. Black key width = 60px. So:
   For black key after white index i (starting at 0 for first white key):
   left = i * 100 + 50 - 30 = i * 100 + 20.
   So:
   i=0: 20px
   i=1: 120px
   i=3: 320px
   i=4: 420px
   i=5: 520px

We need to be careful: after E (i=2) there is no black key. So the i values correspond to the white key index that the black key follows: C# follows C (i=0), D# follows D (i=1), F# follows F (i=3), G# follows G (i=4), A# follows A (i=5). That matches.

Now, we also need to ensure the black keys have appropriate z-index and don't get covered.

Also, we should add a shadow or gradient for a nicer look maybe, but not required.

Now JavaScript:

We need to:

- Get all keys.
- Create synth.
- Set up mapping from key code to note.
- Track pressed keys to avoid repeat.
- Handle mouse events: mousedown, mouseup, mouseleave.
- Handle keyboard events: keydown, keyup.
- Start audio context on first interaction.

Implementation details:

```javascript
document.addEventListener('DOMContentLoaded', async () => {
  // Check if Tone is loaded
  if (typeof Tone === 'undefined') {
    console.error('Tone.js not loaded');
    return;
  }

  // Create synth
  const synth = new Tone.PolySynth(Tone.Synth, {
    envelope: {
      attack: 0.005,
      decay: 1.5,
      sustain: 0,
      release: 2
    },
    oscillator: {
      type: 'triangle'
    }
  }).toDestination();

  // Set volume maybe
  synth.volume.value = -6; // dB

  // Map keys
  const keyElements = document.querySelectorAll('.key');
  const keyToNote = new Map();
  const noteToKey = new Map();

  keyElements.forEach(el => {
    const note = el.getAttribute('data-note');
    const keyChar = el.getAttribute('data-key');
    keyToNote.set(keyChar.toLowerCase(), note);
    noteToKey.set(note, keyChar);
  });

  // Track pressed keys (by key character or note)
  const pressedKeys = new Set();

  // Helper to start note
  function startNote(note, keyChar) {
    if (pressedKeys.has(note)) return;
    pressedKeys.add(note);

    // Start audio context if not started
    if (Tone.context.state !== 'running') {
      Tone.start();
    }

    // visual feedback
    const el = document.querySelector(`.key[data-note="${note}"]`);
    if (el) el.classList.add('active');

    // synth
    synth.triggerAttack(note);
  }

  // Helper to stop note
  function stopNote(note) {
    if (!pressedKeys.has(note)) return;
    pressedKeys.delete(note);

    const el = document.querySelector(`.key[data-note="${note}"]`);
    if (el) el.classList.remove('active');

    synth.triggerRelease(note);
  }

  // Mouse events
  keyElements.forEach(el => {
    const note = el.getAttribute('data-note');
    el.addEventListener('mousedown', (e) => {
      e.preventDefault();
      startNote(note);
    });
    el.addEventListener('mouseup', () => {
      stopNote(note);
    });
    el.addEventListener('mouseleave', () => {
      stopNote(note);
    });
    // Touch support? Could add but not required.
  });

  // Keyboard events
  document.addEventListener('keydown', (e) => {
    const key = e.key.toLowerCase();
    // ignore if it's a modifier or repeat
    if (e.repeat) return;
    const note = keyToNote.get(key);
    if (note) {
      startNote(note, key);
    }
  });

  document.addEventListener('keyup', (e) => {
    const key = e.key.toLowerCase();
    const note = keyToNote.get(key);
    if (note) {
      stopNote(note);
    }
  });

  // Optional: map computer key to note? Already done.
});
```

Potential issues: When using PolySynth, `triggerAttack` and `triggerRelease` are fine. But we need to ensure that if a key is pressed and then another key is pressed, the synth can handle multiple notes. That's fine.

Also, we should handle releasing all notes on unload? Not necessary.

Now, also consider that the ADSR envelope attack and decay times are in seconds. I set attack 0.005s (5ms), decay 1.5s, sustain 0, release 2s. That gives a piano-like sound: quick attack, long decay, no sustain, long release after key release. That should be okay.

But maybe adjust to make it more piano-like: attack ~0.01, decay ~1, release ~1. But it's fine.

Now, final considerations: The piano should be playable. We need to ensure that the black keys are properly clickable. Since they are absolutely positioned and overlap white keys partially, the click event might be caught by the white key if the black key is not higher z-index? Actually black keys have z-index:2, white keys z-index:1, so black should be on top, clickable.

But we must ensure that the white keys don't have pointer events blocking. Both have pointer. It should be fine.

Also, for the active class, we should make sure it's visible. For white keys, active background becomes #ccc. For black, #555.

Now, the instructions: mention which keys to use.

That should cover it.

Now, I'll write the complete index.html file.

I'll ensure the CSS is inside <style> and JS inside <script>. Also include the <script src="tone.min.js"></script> before my JS.

I'll also consider adding a click overlay to start audio because some browsers require a user interaction to start context. But my code already calls Tone.start() on first note start. That should work.

But there is a nuance: Tone.start() returns a promise and must be called in response to a user gesture. The mousedown or keydown event qualifies. So it's fine.

One more thing: I should set the synth's `maxPolyphony` maybe not needed. Also, maybe set `volume` to avoid clipping.

Now, produce the final file.

Let's produce the code.

I'll compute the black key left positions in pixels as above, but I need to exactly compute based on container width. If container width is set to 800px, that's fine. But if I want to be responsive, I could use percentages. But I'll stick with fixed 800px for simplicity. The user can change if needed.

Alternatively, I could use percentages as calculated earlier. But let's do fixed pixel, it's simpler to guarantee correct positions. However, what if the viewport is smaller than 800px? Might overflow. I could set container width: 100%, max-width: 800px. Then the white keys are flex:1 so they fill the container. The black key positions need to be percentages to scale. So I'd better use percentages for responsiveness. Let's switch to percentages.

Let's compute percentages based on 8 white keys. Container width = 100%. Each white key = 12.5% width. Black key width = I'll set to 4%? Actually, if we want the black keys to be about half the width of a white key minus a little, white key width = 12.5%. Half = 6.25%. So black width maybe 4% or 5%. Let's choose 5% for clarity. Then the offset: left = i * 12.5% + (12.5% - 5%)/2 = i*12.5% + 3.75%. Wait: center of gap = (i+1)*12.5%? Actually the boundary between white key i and white key i+1 is at (i+1)*12.5% from left. The black key should be centered on that boundary. So its left = (i+1)*12.5% - (blackWidth/2). But we defined i as the index of the preceding white key. So:
left = (i+1)*12.5% - blackWidth/2.
For i=0: (1*12.5%) - 2.5% = 10%? That would place the black key at 10% left? But earlier I had 4.25% for width 4%. Let's compute properly.

Better: Let's visualize:
- White key i starts at i * (100%/8) and ends at (i+1)*(100%/8). The gap between white key i and i+1 is at (i+1)*(100%/8). We want black key centered at that gap. So black left = gap - blackWidth/2.

For i=0 (C# between C and D):
gap = 1 * 12.5% = 12.5%. Black width = say 6%? Let's pick 6%. Then left = 12.5% - 3% = 9.5%. Hmm that seems high. Actually typical black key is not centered exactly at the gap; it's more towards the right? No, it's centered. But let's see: white key C (i=0) occupies 0-12.5%. D (i=1) occupies 12.5-25%. The boundary at 12.5%. So a black key centered at 12.5% with width 6% would left at 9.5% and right at 15.5%, overlapping both white keys equally. That seems plausible.

But earlier my calculation gave 2.25% which seems too low. That would place black key mostly in the first white key? That seems wrong. Let's recalc using the other formula: left = i * w + w/2 - b/2. That is the same: (i*12.5%) + 6.25% - 3% = i*12.5% + 3.25%. For i=0 gives 3.25%, which is different from 9.5%. Why discrepancy? Because (i+1)*12.5% - b/2 vs i*12.5% + 12.5%/2 - b/2. They are the same: (i+1)*12.5% - b/2 = i*12.5% + 12.5% - b/2. Wait that's not equal to i*12.5% + 12.5%/2 - b/2? Because 12.5% vs 12.5%/2? I made a mistake.

The center of the boundary is at (i+1) * whiteWidth, not at i*whiteWidth + whiteWidth/2? Actually (i+1)*whiteWidth = i*whiteWidth + whiteWidth. That's the position of the right edge of white key i, which is the boundary. The midpoint between the start of white key i and white key i+1 is indeed at (i+0.5)*whiteWidth? Let's clarify:

White key i (0-index) starts at position i * whiteWidth and ends at (i+1)*whiteWidth. So the space between keys i and i+1 is exactly at (i+1)*whiteWidth. There's no "midpoint" between keys; the boundary is that line. The black key should be centered on that boundary line. So its center is at that line. So its left = boundary - blackWidth/2 = (i+1)*whiteWidth - blackWidth/2.

Alternatively, if we think of the left edge of the black key relative to the start of the preceding white key, we can write: blackLeft = i*whiteWidth + whiteWidth - blackWidth/2 = i*whiteWidth + whiteWidth*(1) - blackWidth/2. That is not i*whiteWidth + whiteWidth/2. So my earlier formula using i*whiteWidth + whiteWidth/2 - blackWidth/2 would place the black key centered between the start of white key i and the start of white key i+1? Actually the midpoint between the start of white key i (i*whiteWidth) and start of white key i+1 ((i+1)*whiteWidth) is at (i+0.5)*whiteWidth. That's not the boundary; the boundary is at (i+1)*whiteWidth. That's the right edge of white key i. The black key is usually centered on the line between the two white keys, which is at (i+1)*whiteWidth. So the correct formula is (i+1)*whiteWidth - blackWidth/2.

Thus for i=0: left = 1*12.5% - 3% = 9.5% if blackWidth=6%.
For i=1: left = 2*12.5% - 3% = 25% - 3% = 22%.
For i=3: left = 4*12.5% - 3% = 50% - 3% = 47%.
For i=4: left = 5*12.5% - 3% = 62.5% - 3% = 59.5%.
For i=5: left = 6*12.5% - 3% = 75% - 3% = 72%.

That seems reasonable.

But wait, there is also a black key after B? No, after B (i=6) there is no black key because next is C again. So we only have those five.

So black key left percentages with black width 6%: [9.5%, 22%, 47%, 59.5%, 72%].

But if we choose black width = 5%, then half = 2.5%. Then left = (i+1)*12.5% - 2.5%:
- i=0: 12.5 - 2.5 = 10%
- i=1: 25 - 2.5 = 22.5%
- i=3: 50 - 2.5 = 47.5%
- i=4: 62.5 - 2.5 = 60%
- i=5: 75 - 2.5 = 72.5%

That seems neat: 10%, 22.5%, 47.5%, 60%, 72.5%. That's nice round numbers. So I'll choose black key width = 5% and these left positions.

But careful: The container has padding maybe? We said padding: 10px. That might affect the percentage calculation because percentages are relative to the content box (unless box-sizing: border-box? We set .piano { box-sizing: border-box; } and padding:10px. The width: 100% (or a fixed) includes padding? With border-box, width includes padding. But percentages are relative to the width of the containing block's content box? Actually for absolutely positioned elements, percentage left/right/top/bottom refer to the containing block's padding box? Let's check: The containing block for absolute positioned elements is the nearest positioned ancestor (position: relative). Its padding box is used as the reference for percentages. If .piano has padding:10px, the content width is width - 2*padding. But if we set .piano width:800px; padding:10px; then the content width is 780px. So percentage of left is based on the content width (the padding box). So we need to account for that? Actually the CSS spec: For 'left' and 'right', percentages refer to the width of the containing block's content box. That is the box's width minus padding and border. So if we have container width:800px; padding:10px; border maybe none; then content width = 800 - 2*10 = 780px. So percentages of 800? Actually if we set left:10%, that's 10% of 780px, not 800px. That would cause misalignment if we white keys fill the content area? White keys are flex items inside the content area (since container has padding, the flex container's content area is the padded box). So white keys will occupy the full width of the content area (which is 780px). So if we set white keys flex:1, they will each be 97.5px (780/8). Then the black key left positions should be based on that content width. So if we use percentages, it's okay because percentages are relative to the content width. So we can compute percentages based on content width. That's fine. But our earlier formula used 100% as the total width including padding? Actually we set container width:800px; padding:10px. The content width is 800 - 20 = 780px. So the white keys width = 780px. So if we set black key left as a percentage of the content width, it will align correctly.

But if we set black key left: 10%, that is 10% of 780px = 78px. The boundary between white key i and i+1 is at (i+1)*(780/8) = (i+1)*97.5px. So we need black key centered at those boundaries. That means left = (i+1)*97.5 - (blackWidth/2). But if we also set black key width as a percentage of content width, then we need to compute consistent percentages.

Thus using fixed pixel values might be simpler to avoid percentage miscalculations due to padding. But we can also set container without padding, or use box-sizing: border-box and then set the width to include padding? Actually with box-sizing: border-box, the width includes padding and border. So if we set width:800px; padding:10px; then the total width is 800px (including padding). The content area is 800 - 2*10 = 780px. The flex container's inner space is 780px. The absolute child percentages are relative to the padding box? The spec: The containing block for absolute children is the padding box of the ancestor. Actually for 'left' percentages, they refer to the width of the containing block's content box? I need to recall: For absolutely positioned elements, the containing block is established by the nearest ancestor with a position other than static. The padding box is used as the reference box for percentage calculations for 'top', 'left', 'bottom', 'right'. According to CSS2.1, for absolutely positioned elements, the containing block is the padding box of the nearest positioned ancestor. And the percentages are relative to the width/height of that padding box. So if .piano has position: relative and padding:10px, the padding box's width is the width of the .piano's content area? Actually the padding box includes the padding. So the width of the padding box is the width of the element's box including padding, i.e., the used width (which includes padding). But there is confusion. Let's recall: The CSS box model: content -> padding -> border -> margin. The padding box is content + padding. The border box is content + padding + border. The margin box includes margin. For absolutely positioned elements, the containing block's dimensions are those of the padding box. That means if .piano width:800px (border-box including padding), then the padding box width is the same 800px because the padding is inside. Actually if we set box-sizing: border-box, the width includes padding and border. So .piano width:800px means the total width including padding is 800px. The padding is inside. The padding box then is the area inside the border? Wait: The padding box is the area that includes the content and padding. In border-box, the width property sets the border-box width. So the padding box width would be width - border left - border right? No, that's the content width? Let's be systematic:

Two box models:
- content-box (default): width = content width.
- border-box: width = border-box width (content + padding + border).

But for calculations, when we refer to the padding box, it is the content box + padding. So if we set width:800px border-box and padding:10px (no border), the border-box width is 800px. The padding box width = content width + padding. Since border-box width = content width + padding (if border=0), then padding box width = 800px? Actually content width = border-box width - padding - border. With border-box and no border, content width = 800px - 20px = 780px. Then padding box width = content width + padding = 780px + 20px = 800px. So indeed the padding box width equals the border-box width when there is no border. That's typical: padding box includes padding, so it's larger than content. So in border-box with only padding, the padding box width = the width property. So for absolute child left:%, it uses the padding box width, which is 800px. So if we set left:10%, that's 80px, relative to the padding box width (800px). Meanwhile, the flex container inside .piano has a content area? The flex items are laid out in the content area? Actually the flex container's content box is the area inside padding? The direct children of .piano are flex items. They are laid out within the content area (the area inside padding) if we consider that the flex formatting context is established within the padding box? I think the flex container's available space is the content box (inside padding). Because padding is part of the .piano's padding box but not part of the flex container's content? Actually the .piano itself is the flex container. Its flex items are placed within its content box (the area that excludes padding). The padding is outside the flex container's inner area. So the flex items occupy the content area (width = 800px - 20px = 780px). So the white keys will be 780/8 = 97.5px each. Meanwhile, absolutely positioned children are positioned relative to the padding box (800px). That's a mismatch: They will be offset if we use percentages based on total width but white keys based on content width.

We have two options: 
1) Remove padding from .piano, or use internal container for padding.
2) Use a different method for positioning that aligns with flex items.

Simpler: Don't use padding on the .piano that holds keys. Instead, wrap the keys in an inner container for the visual background, or just set the .piano as flex container without padding, and use separate element for background.

Better: Set .piano as the flex container with no padding, then have an inner .piano-bg or just style the .piano directly but use margin on keys for spacing? Or we can use box-sizing: content-box and set width:800px with no padding. I'll just not use padding on .piano. Instead, I'll give the .piano a border and maybe some margin around the whole piano. If I want some inner spacing, I could use a smaller container inside.

I'll simplify: Remove padding from .piano, set its width to 800px (or 100% max-width 800px). The keys will be flush. I can add a margin on body to center. That avoids padding vs content mismatch.

Alternatively, I could use a container for the keys that has no padding and is positioned relative, while the .piano background element is separate.

Let's go with: .piano { position: relative; width: 800px; height: 200px; background: #111; margin: 20px auto; } No padding. Then the white keys are flex items filling the .piano's content area (which is 800px). So each white key = 100px. Black keys absolute positioned with left pixels: 20px, 120px, 320px, 420px, 520px. Those are based on 100px white key width and 60px black width.

But if we later make width responsive (100% max-width:800px), then we need percentages. But we can make it responsive by using percentages for everything. To avoid calculation headaches, I'll use a fixed pixel width for now. That's acceptable for a demo.

But the user might want a responsive design? Not specified. "build a playable piano keyboard" - not necessarily responsive. I'll go with fixed width for simplicity.

But if I set width: 800px, on small screens it might overflow. I can set max-width: 100% and use vw units? Hmm. Let's set width: 100%; max-width: 800px; and then use percentages for black keys. To use percentages, I need to compute based on the actual content width which will vary. But if I set container width to 100% and white keys flex:1, they'll be equal. For black keys, I need to compute left as a percentage of the container's width. That is independent of padding if I don't have padding. So if container width is 100% of its parent (say body with margin:0), then white key width = (container width)/8. So we can compute black left as (i+1)*(100%/8) - (blackWidthPercent/2). So if blackWidth = 5%, then left = (i+1)*12.5% - 2.5% = (i+1)*12.5% - 2.5%. That's a percentage expression we can put in style directly: left: calc((var(--i) + 1) * 12.5% - 2.5%); But we don't want CSS vars for each. I can pre-calc numbers as above: 10%, 22.5%, 47.5%, 60%, 72.5%. That works if blackWidth=5% and white key width = 12.5%. But is that correct? Let's check: left = (i+1)*12.5% - 2.5%. For i=0: 12.5% - 2.5% = 10%. For i=1: 25% - 2.5% = 22.5%. For i=3: 50% - 2.5% = 47.5%. For i=4: 62.5% - 2.5% = 60%. For i=5: 75% - 2.5% = 72.5%. That matches.

Thus I can set black key style="left: 10%; width: 5%;" etc. That will be responsive.

I'll also need to ensure the black keys have a width that doesn't exceed the gap. 5% seems fine.

Now check: white key width = 12.5%, black width = 5%, so black is 40% of white width, which is a bit narrow but okay visually. Could be 6% -> half 3% -> left = (i+1)*12.5% - 3% = 9.5%, 22%, 47%, 59.5%, 72%. That's also fine. I'll choose 6% for a slightly wider black key. Let's compute left for black width 6%: left = (i+1)*12.5% - 3%. So:
- 12.5-3 = 9.5%
- 25-3 = 22%
- 50-3 = 47%
- 62.5-3 = 59.5%
- 75-3 = 72%

That looks good. I'll use width:6% and these left positions.

Now, need to ensure black key height: 60% maybe of piano height, or fixed? I'll set height: 120px if container height 200px. But with responsive, maybe use vmin? Keep fixed height: 200px. That's okay.

Let's set .piano { height: 200px; } .black { height: 120px; } That's typical.

Now, what about touch support? Not required but nice. Could add touchstart/touchend. But not necessary.

Now, final code.

But wait: The user said "build a playable piano keyboard controlled by mouse and computer keys that synthesizes notes with an ADSR envelope and visual key feedback." So we need to ensure ADSR is applied. In synth config we set envelope: { attack, decay, sustain, release }.

Also, "visual key feedback" – we add active class.

Also, "controlled by mouse and computer keys" – we have both.

Now, let's produce the final index.html.

I will write the complete file. Ensure the HTML is valid and the script for Tone is before our script.

One more thing: Tone.js may need to be loaded from a script tag. In this environment, I assume tone.min.js is present. I will include `<script src="tone.min.js"></script>`.

Now, final code:

I'll produce a clean implementation.

Also, to make the active state more obvious, I'll add a slight transform or border change.

Let's write the code.

I'll include comments maybe but not required.

Potential issue: The black keys might be too wide and overlap neighboring white keys more than desired. But 6% width at 100% container: if container is 800px, 6% = 48px, and white key 100px, so black key centered on boundary will be 48px wide, covering 24px into each adjacent white key. That's okay.

Now, for the HTML of black keys: We need to ensure the order in DOM is after white keys but they are absolute, so order doesn't matter.

Now, test: When clicking a black key, the event should target that black key (z-index higher). Good.

Now, we should also handle keydown repeat: we already check e.repeat. Good.

Also, we should handle multiple keys being pressed simultaneously. Our Set tracks pressed notes, and PolySynth will handle multiple. Good.

But note: When using PolySynth, `triggerAttack` on a note that is already playing (from the same synth) might retrigger the envelope? Actually PolySynth manages voices. If we call triggerAttack on a note that is already sounding, it will treat it as a new note and create a new voice if needed, or if the same note is already playing, it might allow overlapping? For piano, you wouldn't normally re-attack the same note while it's held. Our pressedKeys Set prevents that. But if two different keys, it's fine.

Now, ensuring that on keyup, we call triggerRelease for that note only. PolySynth expects that the note was previously attacked. Should be fine.

Also, we might want to release all notes when the page unloads, but not necessary.

Now, let's produce the final file.

I'll create a full HTML with embedded CSS and JS.

Let's write it.