Bot signals · Web crawler
What is W3C Validator?
W3C validation services
W3C’s free validation services check web content against open standards. Requests come from them when someone uses one to check a page on your site, and W3C says modest traffic from them is not abuse.
01 · Profile
W3C Validator at a glance.
What to look for in your logs, and what the lookup API returns for its addresses.
- Type
- Web crawler
- User agent
W3C_ValidatorValidator.nu/LVW3C-checklinkW3C_I18n-CheckerFeedValidatorW3C_CSS_Validator_JFouffa- robots.txt
User-agent: W3C-checklink- API flag
risk.is_crawler
A user agent alone never sets a flag. Anyone can claim to be W3C Validator; the flag follows the address the request comes from.
02 · In the response
What a match looks like.
A lookup of a W3C Validator address sets risk.is_crawler. The flags come with geolocation and network data in the same response, for API keys on every paid plan.
The response says which kind of bot an address belongs to, not which operator. A request that claims to be W3C Validator while risk.is_crawler is false should not be trusted as W3C Validator.
{
// … location and network fields …
"risk": {
"is_ai": false,
"is_crawler": true,
"is_scanner": false
}
}
03 · Control
Allow or block W3C Validator.
User-agent: W3C-checklink Disallow: /
To enforce it
- robots.txt is a request. Well-behaved bots follow it; impostors using its name don’t.
- Firewall, WAF or CDN. Many offer bot rules you can allow or deny by category. Use them to enforce what robots.txt only asks for.
- In your app. Look up the client address and allow or deny on
risk.is_crawler, alongside the user agent. - Never on the user agent alone. Blocking the name stops only honest clients; allowing it lets anyone in who borrows it.
04 · Questions
About W3C Validator.
What is W3C Validator?
W3C’s free validation services check web content against open standards. Requests come from them when someone uses one to check a page on your site, and W3C says modest traffic from them is not abuse.
How do I block W3C Validator?
Add a robots.txt rule for W3C-checklink. W3C says its Link Checker, as a crawling service, honours robots.txt. Its other validators check a page only when someone submits it. To enforce a block rather than request one, deny it at your firewall, WAF or CDN.
Can I trust a request that says it is W3C Validator?
Not on the user agent alone: any client can send “W3C_Validator” in its user agent. A user agent alone never sets a flag. Check the address with the free bot checker or the lookup API.
Which risk flag does W3C Validator set?
Its addresses set risk.is_crawler in the lookup response. The risk flags come with every paid plan.
From the blog
Go deeper.
Related bots
Recognise W3C Validator on every request.
is_ai, is_crawler and is_scanner in every lookup, on every paid plan, from 9,00 € a month (VAT included).