An alternate style sheet is a CSS file that provides different styling options for a webpage, allowing users to switch between styles. To link an alternate style sheet in HTML, use the `<link>` tag with the `rel` attribute set to "alternate stylesheet" and specify the `title` attribute for identification. For example:
```html
<link rel="stylesheet" type="text/css" href="style1.css" title="Style 1">
<link rel="alternate stylesheet" type="text/css" href="style2.css" title="Style 2">
```
Users can then select the alternate style through a user interface or browser settings.