rel="nofollow" link attribute introduced

What changed

Facing large-scale comment spam on blogs and forums, Google announced a shared link attribute that told crawlers a link should not be treated as an endorsement. The three major engines of the time adopted it simultaneously, which made it an effective de facto standard within weeks. It became the foundation of later link-hygiene guidance and was extended in 2019 with the rel="sponsored" and rel="ugc" values.

Timing

Rollout began 18 January 2005. No completion date was announced. Rankings typically remain unstable for the whole rollout window, so measurements taken inside it are not a reliable read on the outcome.

What it affected