What makes XML invalid?

XML can be invalid if it has missing closing tags, mismatched tag names, incorrect nesting, invalid characters or more than one root element.

Does XML need a schema?

No. XML can be checked as well-formed without a schema. A schema is only needed when you want to validate the document against specific rules.

XML Validator Guide: How To Check and Fix XML Errors

Q: What is XML validation?

XML validation checks whether an XML document is well-formed and, when a schema is used, whether it follows the rules defined by that schema.

An XML validator checks whether your XML is correctly written. It helps find missing tags, broken nesting, invalid characters and schema errors before your XML causes problems in an app, feed or integration.

Last updated: May 2026 Reading time: 6 minutes Reviewed for accuracy

Quick answer:

XML validation checks whether XML is correctly structured.
Well-formed XML follows the basic XML syntax rules.
Schema validation checks XML against rules such as XSD.
Most XML errors come from missing tags, bad nesting or case mismatches.

What is XML validation?

XML validation is the process of checking whether an XML document follows the correct rules.

There are two common levels of checking:

Type	What it checks
Well-formed XML	Checks basic syntax, such as tags, nesting and one root element.
Schema-valid XML	Checks whether the XML follows extra rules defined by a schema such as XSD.

Valid XML example

This XML is valid because the tags match, the structure is nested correctly and there is one root element.

<cleaningTeam>
  <employee>
    <name>Jane Smith</name>
    <role>Senior Floor Cleaner</role>
  </employee>
</cleaningTeam>

Invalid XML example

This XML is invalid because the opening and closing tags do not match.

<name>Jane Smith</Name>

What is wrong?

XML is case-sensitive. <name> and </Name> are not the same tag.

Fixed XML example

The fixed version uses matching lowercase tags.

<name>Jane Smith</name>

Common XML validation errors

Error	Example problem	How to fix it
Missing closing tag	`<title>How to Clean Floors`	Add the closing tag: `</title>`
Case mismatch	`<name>Jane</Name>`	Use exactly matching tag names.
Wrong nesting	`<b><i>text</b></i>`	Close the inner element before the outer element.
Multiple root elements	Two top-level elements in one document	Wrap everything inside one root element.
Invalid characters	Using raw `&` inside text	Escape it as `&`.

Missing closing tag example

A missing closing tag is one of the easiest XML errors to make.

<book>
  <title>How to Clean Floors
</book>

The <title> tag needs a matching </title> closing tag.

Wrong nesting example

XML elements must be closed in the correct order.

<employee>
  <name>Jane Smith
</employee>
  </name>

Correct rule:

The most recently opened tag should be closed first. Think of XML tags like stacking plates: last on, first off.

What is XML schema validation?

XML schema validation checks your XML against a defined set of rules. A schema can specify which elements are allowed, which fields are required and what type of data each field should contain.

For example, a schema might require an employee record to contain a name, a role and an employeeId.

How to validate XML

Paste your XML into an XML validator or formatter.
Check for syntax errors first.
Fix missing or mismatched tags.
Check nesting and root element structure.
If using a schema, validate against the XSD or schema rules.

Validate or format XML online

Use CheeseBridge XML tools to inspect, format and clean up your XML in the browser.

Open XML Formatter Open XML Viewer

Trusted XML references

For official and technical references, see:

Frequently asked questions

What does an XML validator do?

An XML validator checks whether XML is correctly written. It can detect problems such as missing tags, broken nesting, invalid characters and schema rule violations.

Is valid XML the same as well-formed XML?

Not always. Well-formed XML follows basic XML syntax rules. Valid XML can also mean the document follows a schema such as XSD.

Why does XML fail validation?

XML usually fails validation because of missing closing tags, mismatched case, incorrect nesting, invalid characters or schema rule errors.