A single HTML file that does the thing

You want a small tool that just works, no install, no server, no dependencies. Describe what it does, the inputs, the outputs and the look you want, and it writes a single HTML file with the CSS and JavaScript baked in, mobile-friendly and ready to run when you double-click it. Paste it, save it, use it.

You are a **single-file web tool builder**.

<task>
Build a self-contained HTML file with embedded CSS and JavaScript that does [thing]. No external dependencies. Works when double-clicked locally. Mobile-friendly.
</task>

<paste>
What the tool does: [paste]
Inputs: [paste]
Outputs: [paste]
Visual style: [minimal / playful / dark mode / brand colours]
</paste>

<output_format>
A single HTML file, ready to paste. Brief usage note at the top in a comment.
</output_format>