HTML5 Validator

The tool validates HTML5 markup against the WHATWG HTML Living Standard, focusing on syntax, tag matching, attribute validity, doctype correctness, and character encoding. It accepts single fragments or whole documents and reports errors with precise line and column indices, a short code, and contextual guidance to help developers locate and fix issues quickly. It is designed for web developers, QA engineers, and accessibility auditors who need deterministic feedback during development, code reviews, or CI pipelines. By returning machine-readable results in addition to human-friendly messages, it supports automation and traceability across teams.
Core checks include detection of unclosed or misnested elements, missing required attributes, invalid or deprecated attributes, and improper nesting for semantic elements. It flags invalid attribute values, duplicate IDs, and unsafe or non-conforming constructs such as script-injected HTML. Optional lint rules enable stricter enforcement of doctype declarations and explicit meta charset. While it validates static markup, it does not execute scripts or fetch remote resources, ensuring safe validation in environments with restricted network access.
Outputs are designed for automation: a structured JSON report, a human-readable summary, and an export option for CI dashboards. Each issue includes severity, location, a rule identifier, and a suggested fix. The tool adheres to common localization practices and can emit messages in multiple languages if configured. Typical use cases include pre-commit checks, code review QA gates, accessibility audits, and educational demonstrations of correct HTML5 syntax and structure. It differentiates itself through precise location data and machine-friendly results.
Assumes input is text-based HTML; large files may require streaming mode.