If you started building web applications in the last few years, you probably think of APIs as “JSON by default.” That’s a fair assumption: JSON is lightweight, easy to read, and fits naturally with JavaScript-based stacks. Because of that, XML is sometimes labeled as outdated—something you only encounter when maintaining legacy systems.
But XML never disappeared. It simply shifted into roles where its strengths matter most: strict data contracts, document formats, enterprise integration, and metadata-rich messaging. In modern web development, XML is less about being trendy and more about being dependable.
This article explains what XML is, how it works, and why it remains relevant today. You’ll also learn how XML compares to HTML and JSON, where XML is still dominant, and what practical skills help you work with XML confidently.
What Is XML?
XML stands for eXtensible Markup Language. It is a text-based format used to represent structured data. Like HTML, it uses tags and a nested (tree-like) structure. The key difference is that XML is designed for data description and exchange, not for presentation.
The “extensible” part is important. HTML has a predefined set of tags such as <p>, <h1>, and <table>. XML does not. In XML, you create tags that reflect your data model—tags like <customer>, <invoice>, <paymentStatus>, or <shippingAddress>. That flexibility makes XML useful across many domains.
Another way to think about XML is as a “structured text format with strong rules.” Those rules help machines interpret XML consistently and help humans understand what a document contains just by reading tag names.
Core Characteristics of XML
XML has several properties that explain why it is still widely used:
| Feature | Description |
|---|---|
| Self-descriptive tags | Tag names can describe meaning and structure of data. |
| Hierarchical model | XML naturally represents nested relationships (a tree). |
| Platform independent | XML is plain text and works across languages and systems. |
| Unicode support | Supports global characters and multilingual content. |
| Validation options | XML can be validated against schemas (XSD) for strict contracts. |
These characteristics matter most when systems are integrated across organizations, teams, or vendors. In those environments, “clear structure” and “strict validation” can be more important than short payloads.
Well-Formed XML: The Rules That Make XML Reliable
Before XML can be useful, it must be well-formed. That simply means it follows basic XML syntax rules:
- Every opening tag must have a closing tag (or be self-closing).
- Tags must be properly nested (no crossing structures).
- There must be a single root element.
- Attribute values must be quoted.
These rules might feel strict compared to HTML, which browsers often “fix up” automatically. But that strictness is exactly why XML is trusted in machine-to-machine communication. When XML is well-formed, different parsers will interpret it consistently.
XML vs HTML: Different Goals, Different Roles
XML and HTML look similar at first glance because both use angle brackets and nested tags. But they were created for different goals.
HTML is primarily for displaying content. Its tags have predefined meaning in browsers. XML is for describing and transporting data. Its tags have meaning defined by your application or by a standard (like SOAP or RSS).
| Aspect | XML | HTML |
|---|---|---|
| Purpose | Store and transport structured data | Display and format content |
| Tag set | Custom, defined by you or a standard | Predefined tags interpreted by browsers |
| Error tolerance | Strict (must be well-formed) | Lenient (browsers often correct issues) |
| Validation | Common via schemas (XSD) | Not schema-driven in the same way |
A practical way to summarize it: HTML is for humans, XML is for systems. Humans can read XML too, but the primary audience is machines and the primary goal is consistent structure.
XML vs JSON: Why XML Didn’t Disappear
JSON dominates many web APIs because it is simple and lightweight. If you’re building a typical frontend-to-backend API, JSON often feels like the natural choice. So why does XML still matter?
Because XML shines in scenarios where:
- strict validation is required,
- contracts must be stable across many integrations,
- messages need extensible metadata,
- documents are complex and deeply nested,
- enterprise standards and tooling are already built around XML.
| Use Case | XML | JSON |
|---|---|---|
| Frontend REST APIs | Less common | Dominant |
| Strict schemas and validation | Strong (XSD, namespaces) | Possible, but less standardized |
| Enterprise messaging | Very common (SOAP and related standards) | Less common |
| Metadata-rich documents | Excellent support | More limited |
| Payload size | Larger, more verbose | Smaller, compact |
So while JSON is often the best choice for modern public APIs, XML remains a strong choice for systems that prioritize formal structure and long-term compatibility.
XML Schemas and Validation: Why Contracts Matter
One of XML’s biggest advantages is that it can be validated against a schema—most commonly XSD (XML Schema Definition). A schema defines what elements can appear, what attributes exist, the order of elements, and which data types are allowed.
In a complex integration, schemas become a contract. They reduce ambiguity and prevent “guessing” about data structure. That matters when multiple teams or companies integrate systems, or when you have to maintain compatibility across versions.
Validation also improves data quality. If a document fails schema validation, you can reject it early, log the issue, and avoid mysterious downstream failures caused by unexpected fields or missing values.
XML Parsing Models in Web Development
To use XML, you need to parse it. XML parsers generally follow one of three models:
- DOM: loads the entire document into memory as a tree. Easy navigation, higher memory use.
- SAX: streams through the document and fires events. Very memory-efficient, more complex logic.
- StAX: pull-based streaming parser. Efficient like SAX but often more readable.
| Model | Memory | Best For |
|---|---|---|
| DOM | High | Small to medium XML, editing, random access |
| SAX | Low | Large XML streams, high performance processing |
| StAX | Low to medium | Streaming parsing with better control and readability |
In modern web development, DOM might be used for configuration or smaller documents, while SAX/StAX are common in data pipelines and high-volume integrations.
Where XML Still Dominates in Modern Web Development
Even if your day-to-day APIs are JSON, you’ll still encounter XML in several areas. Some are obvious, others surprise developers the first time.
SOAP Web Services
SOAP is a messaging protocol built around XML. Many enterprise and government systems use SOAP because it supports strict contracts (WSDL + XSD) and standardized error handling (SOAP Faults). If you integrate with older platforms or large organizations, SOAP is still common.
RSS and Atom Feeds
RSS and Atom are XML-based formats used for content syndication. While social platforms dominate discovery today, feeds remain important for blogs, news systems, automation tools, and aggregators.
SVG Graphics
SVG (Scalable Vector Graphics) is XML-based. If you work with icons, diagrams, charts, or responsive graphics, you’re already using XML—even if you don’t call it that. SVG is a great example of XML being used for a modern, browser-friendly purpose.
Document Formats (DOCX, XLSX, PPTX)
Many Office file formats are ZIP containers with XML inside (Office Open XML). If your application generates reports, exports spreadsheets, or processes document templates, XML often plays a key role.
Configuration Files and Build Tools
Many ecosystems use XML for configuration: build systems, dependency descriptors, and server configs. Even if newer tools offer alternatives, XML remains common because it is explicit and schema-friendly.
Security and Reliability Considerations
Because XML is used in high-trust integrations, parsing must be done carefully. Security and reliability issues often come from parser configuration rather than from XML itself.
Practical considerations include:
- setting reasonable limits on input size,
- using secure parser defaults and disabling risky features when appropriate,
- validating against schemas when strict contracts are required,
- handling malformed input gracefully with clear error reporting.
In production systems, XML parsing is rarely just “read and hope.” It is typically surrounded by validation, constraints, and logging to ensure predictable behavior.
Advantages of XML in Modern Systems
XML remains relevant because it solves problems that still exist:
Contracts and predictability
XML schemas make data shapes explicit. That reduces surprises and makes integration safer across teams and vendors.
Extensibility without breaking compatibility
With namespaces and structured design, XML allows you to add new elements while maintaining compatibility for existing consumers. This is valuable in long-lived systems that must evolve slowly and safely.
Document-centric workflows
When your payload is more like a document than a simple object (reports, feeds, structured records), XML fits naturally.
Enterprise tooling maturity
Many enterprise platforms have deep tooling support for XML: schema validation, message transformation, routing, governance, and standardized security layers.
Limitations and Criticism of XML
XML is not perfect, and its weaknesses explain why JSON became popular for everyday APIs:
- Verbosity: XML messages can be larger and more repetitive.
- Complexity: namespaces, schemas, and strict rules can slow onboarding.
- Not ideal for simple frontend APIs: JSON is usually faster to work with for typical web app UI data.
| Pros | Cons |
|---|---|
| Strong contracts and validation | Verbose syntax and larger payloads |
| Extensible metadata (namespaces) | More complex parsing and tooling |
| Great for documents and enterprise messaging | Less convenient for typical REST frontend APIs |
When XML Is the Right Choice Today
If you’re building modern web software, XML is still a good choice when:
- you need strict schemas and long-term contracts,
- you integrate with enterprise partners or government systems,
- you process document-like payloads (feeds, structured documents, exports),
- your platform already uses SOAP or XML-based tooling,
- you need metadata-rich messages that evolve safely over time.
When your priorities are simplicity and small payloads—especially for frontend APIs—JSON will often be the better default. Modern development is not about choosing one format forever. It’s about choosing the right format for the job.
Common Misconceptions About XML
“XML is obsolete”
XML is not obsolete—it’s specialized. It remains a standard in many industries and formats that modern software relies on.
“XML is only for legacy systems”
Many current systems generate and consume XML daily. SVG, document formats, feeds, and enterprise integration standards are still active use cases.
“XML cannot scale”
XML can scale when parsed correctly. Streaming parsers and validation strategies exist specifically to handle large payloads and high throughput.
Conclusion
XML is a structured, extensible, and contract-friendly format that still matters in modern web development. While JSON dominates everyday web APIs, XML remains essential for enterprise messaging, document-based workflows, schemas and validation, and standards like SOAP, RSS, and SVG.
If you want to be a well-rounded web developer, XML is worth understanding—not because it replaces JSON, but because it solves a different class of problems. The more integrations and systems you work with, the more likely you are to encounter XML and benefit from knowing how to parse, validate, and reason about it.