The new preview cards now appear when hovering over a link, a feature many websites already have. However, this seemingly simple addition took years to develop due to the complexities of selecting the appropriate thumbnail and generating a summary for millions of pages stored in wikitext. The team faced challenges with algorithms, HTML formatting, and deciding what content to include in the summaries. The process involved extensive collaboration with the editing community and rigorous testing to ensure user privacy and data protection. Despite the challenges, the team is proud of the final product and continues to improve upon it.
https://medium.com/freely-sharing-the-sum-of-all-knowledge/why-it-took-a-long-time-to-build-that-tiny-link-preview-on-wikipedia-d5bd734df8fe