XML allows elements to include attributes, which are name-value pairs placed within the opening tag. These attributes provide metadata or additional context without affecting the element’s main content. For example, <user type="admin">Alex</user> uses the type attribute to describe the user's role while still displaying the name. This offers a dual channel of information—content and attributes—which allows for rich, descriptive data models. JSON, in contrast, does not use attributes. All data is represented using key-value pairs, with no structural separation between metadata and content. While this keeps JSON simpler and more consistent, it limits the way hierarchical metadata can be encoded. To replicate XML's structure, JSON must embed additional keys within objects, which can sometimes make nested data more verbose. XML’s support for attributes can make it more suitable when semantic detail and metadata are critical, such as in configuration files or structured documents.

XML namespaces are a mechanism that allows element and attribute names to be uniquely identified, especially when combining data from multiple XML vocabularies. A namespace is declared using a URI and a prefix, which is applied to elements to avoid name collisions. For instance, in a document using both a weather schema and a calendar schema, <w:date> and <c:date> can coexist, where w and c are prefixes linked to specific namespaces. This system is essential in large-scale XML systems such as SOAP or enterprise services. JSON, however, does not use namespaces because its data is inherently structured using nested objects and keys. Since every key resides within a specific parent object, name collisions are naturally avoided through hierarchy. JSON’s simpler scoping mechanism means there’s no need to declare external URI-based namespaces, making it easier to work with in dynamic applications and avoiding the complexity associated with XML's strict formatting requirements.

Parsing XML introduces several security concerns, particularly when dealing with untrusted or external data. One of the most critical vulnerabilities is XML External Entity (XXE) attacks, where a malicious XML document references external resources, potentially exposing sensitive files or allowing denial-of-service (DoS) attacks. Another issue is Billion Laughs attacks, which involve recursively nested entities that rapidly exhaust system memory during parsing. To mitigate these risks, parsers must be carefully configured to disable external entity resolution and limit recursion depth. JSON is inherently safer because it does not support external entities, document type definitions (DTDs), or complex parsing features that could be exploited. JSON parsers are typically stateless and simpler, reducing the surface area for attacks. However, developers should still validate input, apply size limits, and filter data when necessary. Overall, JSON’s minimal design makes it less vulnerable to structural exploits, while XML requires strict configuration and sanitisation for secure use.

JSON does not natively support comments, as the official specification (ECMA-404) prohibits any content outside of the defined syntax, such as // or /* */. This was a deliberate design choice to ensure that JSON remains strictly data-oriented, eliminating ambiguity and promoting consistent parsing across platforms. XML, by contrast, supports comments using the syntax <!-- comment -->, which can be inserted anywhere in the document for human readability or documentation purposes. Since JSON lacks this feature, developers often work around the limitation in several ways. One approach is to include pseudo-comment fields, such as "__comment": "This section defines user preferences", although this must be removed before data is consumed by applications. Alternatively, comments can be kept in separate documentation, or developers use JSON5 or custom parsers that allow relaxed syntax during development, converting to strict JSON for deployment. While this lack of comment support may hinder documentation, it ensures JSON’s unambiguous parsing behaviour.

XML uses XML Schema Definition (XSD) and other schema languages like DTD (Document Type Definition) to define the structure, data types, and constraints of XML documents. These schemas enforce rules such as element order, allowed attributes, data formats, and required elements, making XML highly self-validating. This is useful in systems that demand strict conformance, such as financial or government data exchange. JSON, on the other hand, can be validated using standards like JSON Schema, which define allowed properties, types, value ranges, and object structures. However, JSON Schema is not natively enforced and requires additional tooling to apply during parsing or processing. While both formats support validation, XML’s tools are more mature and widely standardised, whereas JSON validation is often looser and more flexible. JSON is preferred when rapid development and flexibility are needed, while XML excels in systems where precise control and formal validation are required to maintain data integrity.