What this does
Pretty-prints XML with 2 spaces, 4 spaces, or tabs. The document is first checked with the browser's own XML parser, so malformed input is rejected with a line and column instead of being guessed at.
What is left alone
Entity references like & and custom DTD entities, CDATA sections, the DOCTYPE, processing instructions, attribute quoting, and the content of any element marked xml:space="preserve" are written back exactly as they were. Comments are kept unless you turn them off.
Whitespace
An element that only contains text stays on one line, with its text trimmed. In mixed content such as <p>Hello <b>big</b> world</p> each piece goes on its own line, which can add whitespace between words; use the Minify tab or leave such documents alone if that matters.
Privacy
Everything runs in your browser. Your input is never uploaded, stored, or put in the URL.