Here's a simple HTML app that displays a button. When clicked, it shows an alert with a message. You can copy this code and save it as an `.html` file to run it in your browser.
```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Simple HTML App</title>
<style>
body {
display: flex;
justify-content: center;
