Validator.nu
W3C •
Since 2007
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Official Docs
What is Validator.nu?
Validator.nu (Nu Html Checker) is the W3C's HTML validation service that checks HTML documents for compliance with web standards. It supports HTML5 and is the validator powering the W3C Markup Validator. The tool helps developers identify markup errors, accessibility issues, and potential compatibility problems. Unlike its predecessors, Validator.nu understands modern HTML5 and provides more relevant error messages. It's essential for ensuring web pages are properly structured and accessible.
User Agent String
Mozilla/5.0 (compatible; Validator.nu/LV)
How to Control Validator.nu
Block Completely
To prevent Validator.nu from accessing your entire website, add this to your robots.txt file:
# Block Validator.nu
User-agent: Validator.nu
Disallow: /
Block Specific Directories
To restrict access to certain parts of your site while allowing others:
User-agent: Validator.nu
Disallow: /admin/
Disallow: /private/
Disallow: /wp-admin/
Allow: /public/
Set Crawl Delay
To slow down the crawl rate (note: not all bots respect this directive):
User-agent: Validator.nu
Crawl-delay: 10
How to Verify Validator.nu
Verification Method:
Validator.nu user agent string
Validator.nu user agent string
Learn more in the official documentation.
Detection Patterns
Multiple ways to detect Validator.nu in your application:
Basic Pattern
/Validator\.nu/iStrict Pattern
/^Mozilla/5\.0 \(compatible; Validator\.nu/LV\)$/Flexible Pattern
/Validator\.nu[\s\/]?[\d\.]*?/iVendor Match
/.*W3C.*Validator\.nu/iImplementation Examples
// PHP Detection for Validator.nu
function detect_validator_nu() {
$user_agent = $_SERVER['HTTP_USER_AGENT'] ?? '';
$pattern = '/Validator\\.nu/i';
if (preg_match($pattern, $user_agent)) {
// Log the detection
error_log('Validator.nu detected from IP: ' . $_SERVER['REMOTE_ADDR']);
// Set cache headers
header('Cache-Control: public, max-age=3600');
header('X-Robots-Tag: noarchive');
// Optional: Serve cached version
if (file_exists('cache/' . md5($_SERVER['REQUEST_URI']) . '.html')) {
readfile('cache/' . md5($_SERVER['REQUEST_URI']) . '.html');
exit;
}
return true;
}
return false;
}
# Python/Flask Detection for Validator.nu
import re
from flask import request, make_responsedef detect_validator_nu():
user_agent = request.headers.get('User-Agent', '')
pattern = r'Validator.nu'
if re.search(pattern, user_agent, re.IGNORECASE):
# Create response with caching
response = make_response()
response.headers['Cache-Control'] = 'public, max-age=3600'
response.headers['X-Robots-Tag'] = 'noarchive'
return True
return False# Django Middleware
class ValidatornuMiddleware:
def __init__(self, get_response):
self.get_response = get_response
def __call__(self, request):
if self.detect_bot(request):
# Handle bot traffic
pass
return self.get_response(request)
// JavaScript/Node.js Detection for Validator.nu
const express = require('express');
const app = express();// Middleware to detect Validator.nu
function detectValidatornu(req, res, next) {
const userAgent = req.headers['user-agent'] || '';
const pattern = /Validator.nu/i;
if (pattern.test(userAgent)) {
// Log bot detection
console.log('Validator.nu detected from IP:', req.ip);
// Set cache headers
res.set({
'Cache-Control': 'public, max-age=3600',
'X-Robots-Tag': 'noarchive'
});
// Mark request as bot
req.isBot = true;
req.botName = 'Validator.nu';
}
next();
}app.use(detectValidatornu);
# Apache .htaccess rules for Validator.nu# Block completely
RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} Validator\.nu [NC]
RewriteRule .* - [F,L]# Or redirect to a static version
RewriteCond %{HTTP_USER_AGENT} Validator\.nu [NC]
RewriteCond %{REQUEST_URI} !^/static/
RewriteRule ^(.*)$ /static/$1 [L]# Or set environment variable for PHP
SetEnvIfNoCase User-Agent "Validator\.nu" is_bot=1# Add cache headers for this bot
<If "%{HTTP_USER_AGENT} =~ /Validator\.nu/i">
Header set Cache-Control "public, max-age=3600"
Header set X-Robots-Tag "noarchive"
</If>
# Nginx configuration for Validator.nu# Map user agent to variable
map $http_user_agent $is_validator_nu {
default 0;
~*Validator\.nu 1;
}server {
# Block the bot completely
if ($is_validator_nu) {
return 403;
}
# Or serve cached content
location / {
if ($is_validator_nu) {
root /var/www/cached;
try_files $uri $uri.html $uri/index.html @backend;
}
try_files $uri @backend;
}
# Add headers for bot requests
location @backend {
if ($is_validator_nu) {
add_header Cache-Control "public, max-age=3600";
add_header X-Robots-Tag "noarchive";
}
proxy_pass http://backend;
}
}
Should You Block This Bot?
Recommendations based on your website type:
| Site Type | Recommendation | Reasoning |
|---|---|---|
| E-commerce | Optional | Evaluate based on bandwidth usage vs. benefits |
| Blog/News | Allow | Increases content reach and discoverability |
| SaaS Application | Block | No benefit for application interfaces; preserve resources |
| Documentation | Selective | Allow for public docs, block for internal docs |
| Corporate Site | Limit | Allow for public pages, block sensitive areas like intranets |
Advanced robots.txt Configurations
E-commerce Site Configuration
User-agent: Validator.nu
Crawl-delay: 5
Disallow: /cart/
Disallow: /checkout/
Disallow: /my-account/
Disallow: /api/
Disallow: /*?sort=
Disallow: /*?filter=
Disallow: /*&page=
Allow: /products/
Allow: /categories/
Sitemap: https://example.com/sitemap.xml
Publishing/Blog Configuration
User-agent: Validator.nu
Crawl-delay: 10
Disallow: /wp-admin/
Disallow: /drafts/
Disallow: /preview/
Disallow: /*?replytocom=
Allow: /
SaaS/Application Configuration
User-agent: Validator.nu
Disallow: /app/
Disallow: /api/
Disallow: /dashboard/
Disallow: /settings/
Allow: /
Allow: /pricing/
Allow: /features/
Allow: /docs/
Quick Reference
User Agent Match
Validator.nuRobots.txt Name
Validator.nuCategory
otherRespects robots.txt
Yes
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