XML Formatter, Minifier & Validator

XML (Extensible Markup Language) is a markup format built around nested, explicitly-closed tags — <tag>content</tag> — and it still underpins a huge amount of infrastructure: SOAP APIs, RSS/Atom feeds, SVG images, Android layout files, Microsoft Office's .docx/.xlsx internals, countless enterprise data interchange formats, and configuration for tools like Maven and Ant. Unlike JSON, XML supports attributes on elements, mixed content (text and child elements together), namespaces, and a formal notion of "well-formedness" independent of any particular schema. Because XML is verbose by nature (every element needs both an opening and closing tag), it is very commonly minified for transmission — stripping the whitespace between tags shrinks payload size — and just as commonly pretty-printed for reading, since minified or machine-generated XML (a single line with no indentation) is close to unreadable by eye. This tool does both, along with a validator that reports whether a document is well-formed XML. Rather than implement a custom XML parser — a genuinely hard problem to get fully correct, given namespaces, CDATA sections, processing instructions, entity references and DTD subtleties — this tool uses the browser's own built-in DOMParser and XMLSerializer. Every modern browser ships a battle-tested XML engine as part of its web platform (the same one used to render SVG and parse XHTML), so formatting and validation here inherit that correctness rather than reinventing it. When DOMParser encounters malformed XML, it does not throw a JavaScript exception — instead it returns a document containing a <parsererror> element in place of the content, and this tool detects that element and surfaces its message as a validation error. One practical implication of using DOMParser is that this tool validates well-formedness (are the tags balanced, are attributes quoted, is there exactly one root element, are special characters like & and < properly escaped) rather than schema validity (does the document conform to a specific XSD or DTD). Well-formedness is a property of XML syntax itself and applies to every XML document regardless of what it represents; schema validity is specific to one particular document type and would require the actual schema file, which is out of scope for a general-purpose formatter. If you need to check a document against a specific XSD, you will need a schema-aware validator, but for catching the vast majority of real-world XML mistakes — an unclosed tag, an unescaped ampersand, mismatched tag names, multiple root elements — well-formedness checking is exactly what is needed. Because parsing happens through the browser's DOM, whitespace-only text nodes between element tags (the newlines and indentation of a pretty-printed source) are collapsed during minification, and reinserted at two spaces per nesting level during formatting. Content inside CDATA sections and text content within elements is preserved as-is and is not reformatted, since altering it could change the meaning of the document.