Zero-width characters are invisible Unicode characters like zero-width space (U+200B), zero-width joiner (U+200D), and BOM (U+FEFF). They take no visual space but can affect text processing and often sneak in via copy-paste.
It replaces multiple consecutive whitespace characters (spaces, tabs, newlines) with a single space. For example, "hello world" becomes "hello world".
Yes, absolutely. All processing happens locally in your browser. No text is ever uploaded to any server.