Mobile Readiness: What Mobile-First Indexing Actually Checks

Google indexes the mobile version of your site. The mobile rendering is the one stored, scored and served, to phone and desktop searchers alike. Anything present on desktop and absent on mobile is, for practical purposes, absent. That makes parity the real subject. A responsive site usually passes without effort, because both viewports render from one HTML document. Sites serving separate mobile markup, or adapting server-side by user agent, are where the gaps appear: a truncated title, a dropped meta description, a different H1, a canonical pointing at the desktop URL, or a robots meta that differs between the two renderings. The viewport meta tag is the entry condition. Without it, mobile browsers assume a desktop-width layout and scale it down, which fails usability assessment regardless of how carefully the CSS is written. Server-side adaptation adds one more requirement: a Vary header naming User-Agent, so caches and crawlers know the response changes by device. Without it a CDN can hand a cached desktop response to a mobile crawler. These checks fetch your homepage twice, once as a desktop client and once as a mobile client, then compare the elements search engines actually store.