ValidateHTML

About ValidateHTML

ValidateHTML is a free online tool that validates HTML code against the HTML5 living standard. Users can paste HTML directly or enter a URL to have the page fetched and checked. The tool checks for unclosed and mismatched tags, missing required attributes (such as alt text on images or labels on form inputs), deprecated elements (such as center, font, and marquee), accessibility issues (including missing ARIA labels, empty headings, skipped heading levels, and missing form labels), SEO-related markup (missing DOCTYPE, lang attribute, viewport meta, canonical tags), and attribute or value errors such as duplicate IDs and invalid href formats.

Results are returned instantly and include a quality score from 0 to 100 with a corresponding letter grade (A through F). The score starts at 100 and is reduced by 15 points per unique error type and 3 points per unique warning type, with warnings capped at a 15-point total deduction; repeated instances of the same issue do not cause additional deductions. The tool also provides severity filters, the ability to copy results, and PDF export.

The service is free with no login or registration required and supports unlimited validations of both pasted HTML snippets and live URLs. It is intended for developers checking HTML during development or before deployment, and positions itself as an alternative to the W3C Validator, differentiated by its scoring/grading system and interface, while noting the W3C Validator remains the official standard checker.

Key features

• Validates pasted HTML or a live URL against the HTML5 standard
• Detects unclosed, mismatched, and improperly nested tags
• Flags missing required attributes like image alt text and form labels
• Identifies deprecated HTML elements such as center, font, and marquee
• Checks accessibility issues including WCAG violations and missing ARIA labels
• Generates a 0-100 quality score with letter grade (A-F)

Frequently asked questions