# Html Validator

> HTML Validator is a Mozilla extension that adds HTML validation inside Firefox, Mozilla. The number of errors of a HTML page is seen on the form of an icon For Firefox 56, use 0.974 (see all versions below). For Firefox 57, see Web Extensions.

Canonical page: [https://chrome-stats.com/d/html-validator](https://chrome-stats.com/d/html-validator)

## Overview

- **ID:** `html-validator`
- **Platform:** Firefox
- **Type:** Firefox add-on
- **Status:** Available
- **Publisher:** Marc Gueury
- **Category:** web-development,other
- **Daily users:** 5,960
- **Weekly downloads:** 40
- **Rating:** 4.23 / 5 (26 ratings)
- **Version:** 0.9.9.5
- **Last updated:** 2021-08-04
- **First published:** 2004-09-11
- **Size:** 2.65 MB
- **Data as of:** 2026-09-07
- **Store listing:** [Firefox Add-ons Store](https://addons.mozilla.org/firefox/addon/html-validator/)
- **Website:** [https://www.gueury.com/mozilla/](https://www.gueury.com/mozilla/)

## Description

HTML Validator is a powerful Mozilla Firefox extension that integrates HTML validation directly into your browsing experience. It displays the number of HTML errors through an icon in the status bar or inside the Developer Tools tab, helping web developers quickly identify and address issues.

Built using the trusted Tidy and OpenSP (SGML Parser) algorithms originally developed by the W3C, the extension validates HTML locally without sharing data with third-party servers. It supports validation of server-sent HTML as well as dynamic content after Ajax calls. The latest versions provide comprehensive support for HTML5 and accessibility checks aligned with WAI standards. Now available as a Web Extension compatible with Firefox 57+ and Chrome, HTML Validator is a multilingual tool available in 17 languages with ongoing community support.

## Rankings

- #1,060 — Overall
- #109 — Web Development
- #153 — Other extensions
- #1 — html
- #28 — algorithm
- #28 — x data

## Permissions and access

### Permissions

- `<all_urls>`
- `storage`
- `clipboardWrite`
- `webNavigation`

### Content script matches

- `<all_urls>`

## Safety

- **Risk impact:** High risk impact
- **Risk likelihood:** Low risk likelihood

### Analysis details

- Allows access to all websites, posing a significant security risk as it can monitor and modify data from any visited site.
- This extension has low user count. Unpopular extensions may not be stable or safe.

> Some risk analysis details are omitted from this free response. [Upgrade to view the full analysis](https://chrome-stats.com/pricing).

## Review summary

Users find the Html Validator add-on valuable for catching HTML errors and improving code quality. However, several common issues include loss of support for CSS validation, false warnings in offline mode, broken frame validation, reduced functionality in recent Firefox versions, and less user-friendly access to validation results. Despite these drawbacks, it remains a useful tool for those serious about HTML coding.

### Pros

- Helps users catch HTML errors effectively while browsing or coding, improving code quality.
- Long-standing and reliable tool appreciated by users serious about writing clean HTML.
- Provides validation results directly within the browser, integrating well with developer workflows.

### Cons

- No longer supports CSS HTML Validator, limiting its validation capabilities.
- Offline validation can cause false warnings, such as incorrect missing tags.
- Validation of frames and certain HTML elements is broken or no longer possible in recent Firefox versions.
- Options and compatibility issues with newer Firefox versions and Chrome cause partial functionality or failure.
- Requires extra clicks to access validation results and lacks immediate visual indicators, reducing usability.

## Similar extensions and apps

- [HTML Valid](https://chrome-stats.com/d/html-valid)
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Source: [Chrome-Stats](https://chrome-stats.com/d/html-validator)