Because most searches happen on phones, Google switched to crawling and evaluating the mobile page first. The practical risk is content parity: if your mobile version hides text, images, structured data or internal links that exist on desktop, Google may not see them — which can quietly hurt rankings.
Responsive design is the safest setup, since desktop and mobile share the same HTML. The checklist is straightforward: same primary content on both, the same structured data, accessible images with alt text, and a fast, usable mobile experience.
Mobile-first indexing connects closely to performance and Core Web Vitals, which are measured on mobile. Getting the mobile experience right is now foundational technical SEO, not an optimisation afterthought.
