To ensure valid HTML, we use W3C's validation service on the Wikidata repository.

The current status of the HTML validation is available here. It is generated using this PHP script which is adjusted to run on the dev setup of the Wikidata repository. Variables to adjust inside the script are: $domain (Wikidata repo URL) and $urls (URLs to be validated).

The script uses the Services_W3C_HTMLValidator pear package.