Who should use (X)HTML5 Checker?

(X)HTML5 Checker is a B2B product built for web developers.

Does (X)HTML5 Checker have a free plan?

Yes — (X)HTML5 Checker is free to use.

What platforms does (X)HTML5 Checker run on?

(X)HTML5 Checker runs on the web and embeddable surfaces.

Is (X)HTML5 Checker still maintained?

Unverified. (X)HTML5 Checker has not been re-checked since it entered the index, so there is no finding either way — and only a positive finding would say otherwise.

What projects are similar to (X)HTML5 Checker?

Similar projects tracked by PulseGate include W3C Validator Checker, CSS Validation Checker, and Validate HTML.

Who develops (X)HTML5 Checker?

(X)HTML5 Checker is developed by Francesco De Stefano.

When did (X)HTML5 Checker launch?

(X)HTML5 Checker first shipped in 2018.

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(X)HTML5 Checker

Name: (X)HTML5 Checker
Author: Francesco De Stefano

mozilla.orgBrowser DevTools & extensions

(X)HTML5 Checker is a Firefox WebExtension that sends the current webpage's HTML to the W3C Nu validator service. It displays validation errors in a popup, highlights the affected markup, and shows the relevant source code for refactoring.

FreeWebEmbeddable

Visit mozilla.org

Overview

6 features

In the Browser DevTools & extensions space, (X)HTML5 Checker takes a focused approach. It focuses on checking webpage markup for HTML errors and standards compliance without manually inspecting source code. (X)HTML5 Checker is a B2B product aimed at web developers. It is available for free. (X)HTML5 Checker is available on the web and embeddable surfaces.

Behind (X)HTML5 Checker is Francesco De Stefano, and it first shipped in 2018. Among its 6 catalogued features are HTML validation, error list, and error highlighting. The interface is available in 32 languages, including Czech, German, and Greek. It exposes integrations via a public API.

Summary written by a language model from the project’s public pages.

HTML validation
Error list
Error highlighting
Source display
Popup results
W3C validation

Built with & integrations

Connectors

API

Runs on

BrowserEmbeddable

Trust & compliance

Verified signals

HTTPSPrivacy Policy Terms of ServiceFree tierMulti-languageStore-verified publisher

Legal

Indexing history

What PulseGate has recorded for this listing

Indexed12 Sep · 10:30 UTC
(X)HTML5 Checker verified via Firefox Add-ons
Source: Firefox Add-ons · Open

Frequently asked questions about (X)HTML5 Checker

What is (X)HTML5 Checker?: (X)HTML5 Checker focuses on checking webpage markup for HTML errors and standards compliance without manually inspecting source code. It is catalogued under Browser DevTools & extensions on PulseGate.
Who should use (X)HTML5 Checker?: (X)HTML5 Checker is a B2B product built for web developers.
Does (X)HTML5 Checker have a free plan?: Yes — (X)HTML5 Checker is free to use.
What platforms does (X)HTML5 Checker run on?: (X)HTML5 Checker runs on the web and embeddable surfaces.
Is (X)HTML5 Checker still maintained?: Unverified. (X)HTML5 Checker has not been re-checked since it entered the index, so there is no finding either way — and only a positive finding would say otherwise.
What projects are similar to (X)HTML5 Checker?: Similar projects tracked by PulseGate include W3C Validator Checker, CSS Validation Checker, and Validate HTML.W3C Validator Checker CSS Validation Checker Validate HTML
Who develops (X)HTML5 Checker?: (X)HTML5 Checker is developed by Francesco De Stefano.
When did (X)HTML5 Checker launch?: (X)HTML5 Checker first shipped in 2018.

At a glance

Pricing: Free
Platforms: Web
First seen: 12 Sep 2026
Built for: web developers
Model: B2B
Solves: Checking webpage markup for HTML errors and standards compliance without manually inspecting source code.

Registered as

Firefox Add-ons: x-html5-checker

Developer

Francesco De Stefano

Index record

Indexed: 12 Sep 2026
Lifecycle: Alive
First seen: Sep 2026
Last seen: 12 Sep 2026

Identity audit (13)

Slug: x-html5-checker-mozilla-org
Lifecycle state recorded: 12 Sep 2026
Verification state: Indexed for public listing
Listing state: Listed: yes
Index status: Included in index
Latest evidence snapshot: 12 Sep 2026
Timeline basis: Indexed-at chronology (no inferred launch/funding milestones).
Name from: Derived from the project's own page and URL.
Category from: Assigned by a language model.
Summary from: Written by a language model from public pages.
Languages from: Detected by a language model, checked against the page's own declaration.
Last updated: 12 Sep 2026
Canonical URL: https://addons.mozilla.org/en-US/firefox/addon/x-html5-checker

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