XML Formatter

No. The formatter runs entirely in your browser — the document never leaves the page, and there is no server involved in the version you are using on screen. That is deliberate rather than incidental: the XML most worth formatting is usually a config file, an invoice or an API response, none of which should be pasted into an unknown server to add some line breaks.

Well formed means the syntax holds together: every tag closes, tags nest rather than overlap, attributes are quoted, and constructs like comments and CDATA are properly terminated. Valid means it also matches a schema — a DTD or XSD saying which elements may appear where, and what they may contain. This tool checks the first. A document can be perfectly well formed and still be nonsense to the system expecting it.

Yes, both, and this is worth checking in any formatter you use. Comments are kept by default and can be dropped with a switch if you are minifying for production. CDATA blocks are passed through untouched, including whitespace inside them — a formatter that reindents the contents of a CDATA block has changed your data, not your formatting. The XML declaration, doctype and processing instructions are all preserved too.

Because a closing tag names a different element from the one still open — the classic being tags that overlap rather than nest, like an element opened inside another and closed outside it. The message names the line of the first mismatch, which is usually a few lines after the real mistake: the tag that was never closed is the culprit, and the error surfaces where the next close arrives and finds the wrong thing waiting.

It handles whatever your browser can hold in memory, which in practice means multi-megabyte documents are fine and a very large export may be slow while you type. There is no server-side limit because there is no server. If you are formatting something enormous, the API is the better route — it takes the same options and does the work in one request rather than on every keystroke.