The user wants me to create a single-page web app with a procedural L-system fractal tree that branches recursively and sways gently in the wind. I need to use the pre-provided p5.min.js library.

Let me create an index.html file with:
1. CSS in a <style> tag
2. JS in a <script> tag
3. Load p5.min.js from the current directory
4. Implement an L-system fractal tree with wind animation

An L-system tree typically uses rules like:
- F = draw forward
- + = turn right
- - = turn left
- [ = save state
- ] = restore state

For a tree, common rules are:
- Axiom: F
- Rule: F -> FF+[+F-F-F]-[-F+F+F]

Or simpler:
- Axiom: X
- Rule: X -> F+[[X]-X]-F[-FX]+X
- Rule: F -> FF

Let me create a beautiful procedural tree with gentle wind sway.

Done! I've created a self-contained `index.html` file with a procedural L-system fractal tree that:

1. **L-System Grammar**: Uses the axiom "X" with rules that create complex branching patterns
2. **Recursive Branching**: The L-system sentence is interpreted to draw branches recursively with push/pop for branch states
3. **Wind Animation**: Branches sway gently using sine waves based on depth in the tree structure
4. **Visual Features**:
   - Gradient night sky with stars and moon
   - Color gradient from trunk brown to leaf green based on branch depth
   - Leaves drawn at branch ends
   - Falling leaves animation
   - Ground plane

5. **Interactivity**:
   - Click to regenerate with new random parameters
   - Press 'S' to save the canvas as PNG

The tree sways organically with wind affecting branches differently based on their depth in the hierarchy.