Format re-indents HTML by putting each opening tag, closing tag and text node on its own line and nesting them by depth. It understands void elements such as br, img, input and meta so they never get a bogus closing tag, and it leaves the contents of pre, script, style and textarea untouched so preformatted text and code are not reflowed.
Minify strips the insignificant whitespace between tags and comments to produce a compact single-line document, again leaving pre, script and style content intact. A small hand-written tokenizer walks the markup, so no external library is used and the result is deterministic. Everything runs locally in your browser — your markup is never uploaded.