Developer Guides
What Validators Catch, and What They Miss
Syntax and format checks find broken markup, bad JSON and mistyped numbers, but not meaning, business rules, email deliverability or whether a URL is alive.
Try it: HTML Validator
Validators
A validator checks something against a rule before anything depends on it: markup with an unclosed tag, a stylesheet browsers will quietly ignore, an email that can never deliver, or a card number with a mistyped digit. These validators explain what failed and where, and they run entirely in your browser.
Checked in your browser
Code, email addresses, card numbers and passwords are validated on your device and are not uploaded to a server.
Errors point to the exact spot
HTML and CSS issues list a line, XML reports where parsing stops, and JSON Schema errors name the failing path and the rule it broke.
Covered by automated tests
The format, HTML, CSS and JSON Schema validators are tested with valid and invalid examples, including edge cases such as IPv6 shortcuts and leap years.
Free, no sign-up
Validate as many strings and files as you need without creating an account.
The tools most visitors start with, and what each one is for.
Validate HTML code for syntax errors and compliance.
Find unclosed and mis-nested tags, duplicate ids and missing alt, doctype, title or lang before a page goes live.
Validate JSON data against JSON Schema specifications.
Check a JSON document against a schema and see which field fails and why, for Draft-07, 2019-09 or 2020-12.
Validate CSS code for syntax errors and compliance with W3C standards.
Catch unclosed braces, missing semicolons and invalid values that make browsers silently drop CSS rules.
Validate email addresses for correct format and syntax.
Check an address against standard syntax rules and get a suggestion when a domain looks like a typo.
Validate URLs for correct format and structure.
Confirm a web address is well formed and see its scheme, host, port, path, query and fragment broken out.
Validate XML documents for well-formed syntax and structure.
Confirm a feed, config or data export is well-formed XML and find the exact line where parsing breaks.
Validate UUID and GUID strings for correct format and version.
Verify a UUID or GUID and read its version, variant and, for time-based versions, the embedded timestamp.
Validate IP addresses for correct IPv4 and IPv6 format.
Check an IPv4 or IPv6 address before it goes into a firewall rule, allow-list or config file.
Validate credit card numbers using Luhn algorithm.
Catch a mistyped card number with the Luhn check and identify the issuer network, using test numbers while developing.
Check how strong your password is. Instant analysis with improvement tips.
Rate a password on length, character variety, repeats and simple sequences, with tips to strengthen it.
Short explainers that show how the maths or the file format works, and which tool to reach for next.
Developer Guides
Syntax and format checks find broken markup, bad JSON and mistyped numbers, but not meaning, business rules, email deliverability or whether a URL is alive.
Try it: HTML Validator
Security Guides
Why a long random password beats a short complicated one, how entropy is calculated, and how long guessing actually takes.
Try it: Password Generator
Each group explains what its tools are for, so you can find the right one without scrolling a long list.
For files you write and publish: HTML structure, CSS rules, XML well-formedness and JSON checked against a schema. These find the mistakes that break pages, feeds and API calls, with line numbers or field paths to help you fix them.
For strings that must be in an exact shape to work: email addresses, URLs, IPv4 and IPv6 addresses, MAC addresses, UUIDs and Base64. Use them before data goes into a form, database, allow-list or request.
For values with a built-in checksum or calendar rule: credit card numbers, ISBN-10 and ISBN-13 codes, dates that must be real, and hex color codes that must have the right digits before you use them in CSS.
A local strength check for passwords, based on length, character variety, repeated characters and simple sequences. It runs in your browser, so the password you type is never transmitted.
Browsers quietly ignore broken CSS and forgive sloppy HTML, so problems hide until they hurt. A validator lists them while they are still cheap to fix.
The UUID validator reports version and variant, the IP validator flags leading zeros and ranges, the URL validator splits the address into parts, and the ISBN validator shows the ISBN-10 equivalent.
Format checks cannot know whether a mailbox exists, a card is active, a book was published or a page is online. Each page says what it does not test.
HTML optional end tags are not flagged, UUIDs follow RFC 9562, card numbers use Luhn plus issuer ranges, and JSON Schema drafts are chosen from the schema itself.
You can paste a card number for testing, an internal URL or a draft config without sending it anywhere, because the checks run locally.
Each validator follows the published rule for its format: the Luhn checksum and issuer ranges for cards, the ISBN check-digit formulas, RFC 9562 for UUID versions, IPv4 octet limits and IPv6 compression rules, and the JSON Schema drafts for schema checks. The HTML checker understands optional end tags. The CSS checker parses structure itself and asks the browser's CSS engine to judge values. XML uses the browser's parser. Automated tests cover valid and invalid examples for the format validators, HTML and CSS linting, and JSON Schema, and every page states what it does not check, such as whether an address exists.
No. The Email Validator checks syntax only: the local part, the @ sign and the domain labels, plus common domain typos. It cannot tell whether a mailbox exists or can receive mail. The only reliable test is sending a confirmation email and asking the person to click it.
No. The Luhn checksum catches most single-digit typos and swapped neighbors, and the tool checks the issuer prefix and length, but it never contacts a bank. A well-formed number may not exist or be active. While developing, use the published test numbers from your payment provider.
Well-formed XML follows the syntax rules: every tag closed and nested, one root element and quoted attributes. Valid XML also conforms to a schema or DTD. The XML Validator checks well-formedness using your browser's XML parser and does not check a document against a schema.
The tool parses the stylesheet structure itself but asks your browser's own CSS engine to judge property names, values, selectors and media queries. Very new or vendor-specific features that your browser does not support are reported as unsupported, so try another up-to-date browser before changing correct code.
Parsing only proves the syntax is valid. A schema checks meaning: field types, required properties, allowed values, patterns and formats such as email or date-time. Each error names the failing path. The Draft-07, 2019-09 or 2020-12 rules are chosen from the $schema value, and remote $ref URLs are not fetched.
It flags it as ambiguous rather than guessing, because March 4 and April 3 are both real dates. It understands ISO, US, day-first and written formats, and handles leap years and month lengths. Use the ISO form, 2026-04-03, to remove any doubt.
No. The checker scores length, character variety, repeated characters and simple sequences. It does not compare the password against lists of breached or common passwords, so a reused or predictable phrase can still score well. Use a unique password for every account and a password manager.
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