Have you ever tried to validate a web page at the W3C validator and faced a warning message like this?
Sorry! This document can not be checked.
*Excessive traffic pattern blocked*
Due to excessive traffic we are blocking a pattern from specific browser plugins. If you are using a browser plugin that checks every single page you visit, please discontinue using it and only use selectively.
Please use the form interface on the Validator Home Page to check the page by URL.
If you have, you've been banned.
The W3C validator has added you to a black list, and you're no longer allowed to validate web pages on their site. You can read the warning on their official site:
"Please be considerate in using this shared, free resource. Consider Installing your own instance of the validator for smooth and fast operation. Excessive use of the W3C Validation Service will be blocked."
Why have I been banned?
In most cases, this happens due to excessive traffic from you. Basically, you're doing too many validations, or you're doing them too fast. So, the W3C validator server has detected this pattern and has added your IP address to its block list.
Maybe you're not aware that you're doing so many validations. In most cases, this happens after installing some software on your computer to automate validations at the W3C, like a browser plugin. Many of these browser plugins, add-ons or extensions are too aggresive, and they validate every single web page that you visit. You surely have installed it to validate a site you're developing, and you were not aware that this browser plugin continued validating every single page you visited afterwards.
A quick search of "chrome w3c extension" will give you a lot of browser plugins like these but, for example on this one the developer has put a notice about "NOT WORKING ANYMORE... W3C Validator has blocked the plugin for now... trying to find a solution...".
OK, I understand, how do I fix that?
Fine, once you understand that the W3C validator has blocked your IP due to excessive traffic, it's time to fix this.
First, uninstall any browser plugin / add-on / extension or any other software that may be doing all these excessive automated validations. Also, if you're sharing your internet connection with others (say, in your office), ensure everyone else uninstalls those as well.
Then, choose validation software or services that only make the validations you need. Some browser plugins have a manual mode, and of course our site-wide validator only validates the sites you request.
As for the block itself, it will clear automatically after the excessive traffic from your side stops.
Using alternative validation servers
You can also consider installing the W3C validator software on your own servers. If you have basic sysadmin skills, you can follow these guides to do it:
https://github.com/tlvince/w3c-validator-guide
Or, you can use a prepared Docker image that contains the Validator.nu service.
This way, you'll have the same validation software that the W3C validator has (as it's open source), but as it's on your own server, you don't have the traffic limitation that the W3C systems team imposes.
That's what we do at Rocket Validator.
To be able to offer site-wide validation to all of our clients, we maintain our own dedicated servers, so none of the validations you make through our service passes through the W3C servers.