做个页面

Make me a page with create_page: {{#input}}{{input}}{{/input}}{{^input}}a small, useful tool; first ask me in one line what it should do{{/input}}.
One self-contained HTML document: inline CSS and JavaScript, nothing loaded from the web (no libraries, CDNs, fonts, images or requests: the page runs offline), so any data and pictures (inline SVG) go in the file; links to websites as plain <a href> links.
Make it work on the first try: a clean layout that fits a browser tab, readable in light and dark (prefers-color-scheme), usable with the keyboard, no placeholder features. No eval.
Then tell me in two lines what it does, and that I can ask for changes (you make them with update_page).