Mobile-First Indexing.

Mobile-first indexing means Google primarily uses the mobile version of a website's content for indexing and ranking. Introduced in 2016 and completed for all new websites by 2020 and existing sites by 2021, mobile-first indexing reflects the reality that the majority of Google searches now happen on mobile devices. If a site's mobile experience is degraded, slower, missing content, or fundamentally different from the desktop version, rankings suffer.

Mobile-First Indexing Requirements

For mobile-first indexing, websites must: ensure the mobile version contains all the same content as the desktop version (hidden accordions and tabs are fine, but content must still be present in the HTML); use the same metadata (title tags, meta descriptions, structured data) on both versions; have equivalent internal linking on both versions; and serve fast, functional experiences on mobile (Core Web Vitals are measured from mobile user data in most markets).

Responsive vs. Separate Mobile Sites

The recommended approach for mobile-first is responsive design: one URL serving adaptive content across all device sizes. Separate mobile sites (m.example.com) require correct bidirectional annotations and content parity maintenance, and dynamic serving (detecting device and serving different HTML) requires correct Vary header implementation. Responsive design eliminates these complexity risks and is Google's preferred implementation.