What are DTD and XSD?

DTD (Document Type Definition) is the original XML schema language, defined in the XML 1.0 spec. It defines allowed elements, attributes, and entity references, but has limited data type support (no number ranges, regex patterns, or conditional validation). XSD (XML Schema Definition, W3C, 2001) is the modern replacement — it supports 44 built-in data types (string, integer, date, etc.), namespace-aware validation, complex type inheritance, and fine-grained occurrence constraints. XSD itself is written in XML, while DTD has its own non-XML syntax. Most modern enterprise systems use XSD over DTD.

Can the formatter handle very large XML files?

Yes. The streaming parser handles files up to 50 MB in the browser. For very deep nesting (100+ levels), the tree view paginates nodes for performance. For files over 50 MB, the "Preview" mode parses the first 10,000 elements for formatting while reporting the full document's element count and structure. For production-scale XML processing, pair our formatter with command-line tools like xmllint (libxml2) or Saxon (XSLT/XQuery processor).

What is XPath and how do I use it?

XPath is a query language for selecting nodes from an XML document. Think of it as SQL for XML, or CSS selectors for XML trees. Basic XPath expressions: /catalog/book (all book elements directly under catalog), //book (all book elements anywhere), /catalog/book[1] (the first book), /catalog/book[@category="fiction"] (books with a category attribute equal to "fiction"), /catalog/book[price>30]/title (title elements of books where the price child element is greater than 30). Our XPath evaluator supports XPath 1.0 with real-time highlighting of matching nodes.

Why is XML still used when JSON is available?

XML persists in domains where its unique features are necessary: (1) formal schema validation with XSD — critical for financial, legal, and healthcare data interchange where data integrity is mandatory; (2) namespaces — essential when combining multiple XML vocabularies in one document; (3) mixed content — representing text with inline markup (e.g., "the quick brown fox"), which JSON cannot natively represent; (4) a mature ecosystem of tools — XSLT for transformation, XPath for querying, XSD for validation, and XQuery for database-like operations on XML collections.

How do I debug a SOAP API response?

Paste the raw SOAP XML response into our formatter. The tool detects the SOAP envelope, expands namespace prefixes for readability, and highlights the Body content. If the response is a SOAP fault, the element is flagged in red with the faultcode (e.g., soapenv:Client for a bad request, soapenv:Server for a server-side error) and faultstring (human-readable error description). Check the faultactor element to identify which SOAP intermediary node generated the fault.

Developer Tools2026-05-10

XML Formatter: Beautify, Validate, and Debug XML Online

XML (Extensible Markup Language) remains the backbone of enterprise systems, SOAP APIs, document formats (DOCX, ODS, SVG), and configuration files — even three decades after its W3C recommendation in 1998. But raw XML is notoriously hard to read: a deeply nested document with no line breaks, mixed namespaces, and CDATA sections is a developer's readability nightmare. Our XML formatter beautifies any XML document with configurable indentation (2 or 4 spaces), syntax highlighting, and collapsible tree navigation. The validator checks well-formedness (proper tag matching, attribute quoting, character escaping) and optionally validates against a DTD or XSD schema. Unlike JSON, XML has a formal concept of "well-formed vs valid" — a document can be well-formed (parseable XML syntax) but still invalid (fails schema constraints). The tool also includes an XPath 1.0 evaluator for querying elements and attributes from large XML documents without writing code, and a SOAP envelope formatter that handles the SOAP-specific namespaces and structure commonly encountered in enterprise API debugging. All processing happens in your browser — your XML configuration files, API responses, and schema documents never leave your machine.

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XML Formatter

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Step-by-Step Guide

Paste or Upload Your XML

Paste XML directly into the editor, or upload a .xml file. The formatter accepts any XML variant — SOAP envelopes (soapenv:Envelope), RSS/Atom feeds, XHTML, SVG, or custom application XML. The editor includes line numbers and highlights syntax errors in red as you type, with the error description shown in a panel below.

Format and Optionally Validate

Click "Format" to beautify the XML with your chosen indentation (2 or 4 spaces). The tree view on the right shows the document structure with expandable/collapsible nodes — clicking any node scrolls the editor to that element. For validation, enable "Validate against schema" and upload a DTD or XSD file; schema violations are reported with line-level precision and an explanation of what the schema requires.

Query with XPath and Export Results

Enter an XPath 1.0 expression (e.g., /catalog/book[price<30]/title) to query the document. Matching nodes are highlighted in the tree view and listed in the results panel. Copy the formatted XML, download it as a .xml file, or export XPath query results as JSON for further processing in your application.

Tips & Best Practices

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A well-formed XML document must follow five rules: (1) a single root element contains all other elements; (2) every opening tag has a matching closing tag (or is self-closing, e.g., <br/>); (3) tags must be properly nested — <a><b></a></b> is illegal; (4) attribute values must be quoted (single or double quotes); (5) the five predefined entities (&, <, >, ', ") must be used for reserved characters.

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XML namespaces (xmlns attributes) prevent element name collisions when combining XML from multiple vocabularies. For example, a single XML document might include both XHTML elements (xmlns="http://www.w3.org/1999/xhtml") and SVG elements (xmlns:svg="http://www.w3.org/2000/svg"). The namespace prefix (e.g., svg:) is a local alias; the actual identity of the element is determined by the namespace URI plus the local name.

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CDATA sections (<![CDATA[ ... ]]>) let you include text that contains characters that would otherwise need escaping (like <, >, and &). They are commonly used for embedding code snippets, HTML fragments, or JSON inside XML without escaping every special character. However, CDATA cannot contain the string "]]>" (the CDATA end marker), and it cannot be nested.

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XPath is XML's query language — like SQL for XML data. Common expressions: /root/element (absolute path), //element (find all elements anywhere in the document), /root/element[@attr="value"] (filter by attribute), /root/element[position()<=3] (first three elements), /root/element/text() (extract text content). XPath 1.0 is supported by virtually every XML parser; XPath 2.0 and 3.0 add functions, data types, and conditional expressions but have less universal support.

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XML vs JSON trade-off: XML has a formal schema language (XSD), namespaces, comments, processing instructions, and mixed content (text interleaved with child elements) — features JSON lacks or handles through ad-hoc conventions. JSON is lighter, maps directly to programming language data structures, and is faster to parse. Choose XML for document-centric data with complex structure and formal validation requirements (legal documents, financial reporting, health records); choose JSON for API data interchange between services.

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SOAP (Simple Object Access Protocol) APIs wrap their payload in an XML envelope with standardized elements: <soapenv:Envelope>, <soapenv:Header> (optional — authentication, transactions, routing), and <soapenv:Body> (the actual request/response data). Debugging SOAP errors often involves inspecting the <soapenv:Fault> element inside the Body. Our formatter detects SOAP envelopes and applies SOAP-specific formatting with namespaces expanded.

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RSS 2.0 and Atom are XML-based web feed formats. RSS requires a <rss version="2.0"> root element with a single <channel> containing <item> elements. Atom uses an <feed> root with <entry> children. Both formats are widely used for podcasts, blog syndication, and news aggregation. Our formatter validates feed-specific required elements (e.g., <title>, <link>, <description> in RSS; <id>, <title>, <updated> in Atom) and highlights missing or malformed fields.

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Processing instructions (<? ... ?>) are instructions to the XML processor that are not part of the document's character data. The most common is the XML declaration at the start of the file: <?xml version="1.0" encoding="UTF-8"?>. Others include <?xml-stylesheet?> for linking CSS and <?php ... ?> for embedding PHP code. Processing instructions are preserved during formatting.

Frequently Asked Questions

Well-formed XML follows the basic syntax rules: proper nesting, quoted attributes, matching tags, and correct entity escaping. Any XML parser can parse well-formed XML. Valid XML goes a step further: it is well-formed AND conforms to a schema (DTD or XSD) that defines the allowed elements, attributes, their order, data types, and cardinality. A well-formed XML document can still be invalid — for example, having a <price> element where the schema requires an integer but the document contains "free" as the value.

XML powers the enterprise — from SOAP APIs to document formats to configuration management. Our free formatter makes XML readable, validates it against schemas, and lets you query it with XPath — all in your browser, all private. Paste your XML and see it transform into a clean, navigable structure.

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