Google SEO
2016 (rollout from 2018) — Ahrefs · Google Search Central
What did the mobile-first indexing announcement mean?
Google announced that it would primarily index and rank using a website's mobile version. Content, markup and internal links therefore need to be equivalent to, or better than, the desktop version.
Further analysis
Use this entry as an audit checkpoint. The cited source establishes the context; the direct answer identifies what to check before choosing an action.
In SFT, this topic relates to Structure. Check whether the signal is visible on the page, connected to the rest of the website and supported by understandable evidence.
Related search terms include mobile-first indexing, 2016. Use them to describe the issue in a brief, audit or answer page.
Applying SFT
Structure turns a rule into readable architecture: headings, URLs, structured data, visible content and clear hierarchy.