What HTML errors does ValidateHTML check for?

ValidateHTML checks for unclosed tags, missing required attributes, deprecated HTML elements, incorrect tag nesting, missing DOCTYPE, invalid attribute values, accessibility issues, and more.

Is ValidateHTML free to use?

Yes, ValidateHTML is completely free. You can validate unlimited HTML code snippets and URLs at no cost. No registration required.

How is the HTML quality score calculated?

Your HTML quality score starts at 100. Each error deducts 10 points and each warning deducts 3 points. Grades: A (90-100), B (75-89), C (50-74), D (25-49), F (below 25).

Does HTML validation affect SEO?

Yes. Clean markup improves crawlability, reduces rendering errors, and ensures search engines can properly parse your content. Valid HTML also improves accessibility, which is increasingly important for SEO.

JSON Validator& Formatter

Paste your JSON to validate syntax, find errors with exact positions, and get beautifully formatted output in one click.

Input JSON

Ctrl+Enter to validate

Formatted Output

Instant Validation

Validate your JSON in milliseconds. Get clear error messages with exact line and column numbers to fix issues fast.

Auto Formatting

Valid JSON is automatically formatted with proper indentation. Compare input and output size side by side.

Copy & Share

Copy formatted JSON with one click. Perfect for cleaning up API responses, config files, and data payloads.

What Is JSON?

JSON (JavaScript Object Notation) is a lightweight data interchange format. It is easy for humans to read and write, and easy for machines to parse and generate. JSON is the most widely used format for APIs, configuration files, and data storage on the web.

A valid JSON document must follow strict syntax rules: strings must use double quotes, there are no trailing commas, keys must be quoted, and only specific value types are allowed (strings, numbers, booleans, null, arrays, and objects). Unlike JavaScript, JSON does not support comments, single quotes, or undefined values.

Because JSON syntax is strict, even a single misplaced character will make the entire document invalid. A JSON validator helps you find these issues instantly, saving time when debugging API responses, configuration files, or data exports.

Common JSON Errors

Trailing Commas

JSON does not allow a comma after the last item in an array or object. [1, 2, 3,] is invalid. Remove the trailing comma.

["a", "b", "c",]  // invalid
["a", "b", "c"]   // valid

Single Quotes

JSON requires double quotes for strings and keys. Single quotes are not valid. 'hello' must be "hello".

{'name': 'John'}  // invalid
{"name": "John"}  // valid

Missing Quotes on Keys

All keys in JSON objects must be double-quoted strings. Unquoted keys are valid in JavaScript but not in JSON.

{name: "John"}    // invalid
{"name": "John"}  // valid

Comments

JSON does not support comments of any kind. Neither // single-line nor /* multi-line */ comments are allowed.

{
  // name
  "name": "John"
}  // invalid

Unescaped Special Characters

Strings containing backslashes, quotes, or control characters must be properly escaped with a backslash.

{"path": "C:\Users"}  // invalid
{"path": "C:\\Users"} // valid

Wrong Boolean/Null Casing

JSON booleans and null must be lowercase: true, false, null. True, False, None, NULL are all invalid.

{"active": True}   // invalid
{"active": true}   // valid

Frequently Asked Questions

What does a JSON validator check?

A JSON validator checks that your JSON follows the correct syntax: properly quoted keys and strings, valid data types, correct nesting of objects and arrays, no trailing commas, no comments, and no single quotes. If any rule is broken, the validator reports the exact location of the error.

What is the difference between JSON and JavaScript objects?

JSON is a stricter subset of JavaScript object syntax. In JSON, all keys must be double-quoted strings, all string values must use double quotes, trailing commas are forbidden, comments are not allowed, and only specific value types (string, number, boolean, null, array, object) are permitted. JavaScript objects are more flexible and support all of these.

Why is my JSON invalid even though it works in JavaScript?

JavaScript is more lenient than JSON. Common issues include using single quotes instead of double quotes, having trailing commas, using unquoted keys, including comments, or using JavaScript-specific values like undefined or NaN. All of these are valid JavaScript but invalid JSON.

How do I fix a JSON parsing error?

First, use a JSON validator to find the exact line and column of the error. Common fixes include: replacing single quotes with double quotes, removing trailing commas, quoting all keys, removing comments, escaping special characters in strings, and using lowercase true/false/null.

Is this JSON validator free to use?

Yes, our JSON validator is completely free with no limits or registration required. Paste your JSON, validate it, and get formatted output as many times as you need.

Can I validate large JSON files?

Yes. Our validator can handle large JSON documents. For very large files (over 1MB), validation may take a moment, but there is no hard limit. The formatted output preserves the full structure with proper indentation.

What is JSON Lint?

JSON Lint is the process of validating JSON syntax, similar to how a code linter checks programming languages for errors. Our JSON validator acts as a JSON linter, checking syntax and providing clear error messages with exact positions.