Mobile-first indexing
Google crawls and ranks the phone version of your page, so anything missing on mobile is missing from search.
See it
What it is
Google crawls with a smartphone Googlebot and indexes what that crawler sees. The desktop rendering of your page is no longer the reference copy for anything, and this is not an opt-in setting: it applies to every site. If content, headings, structured data, or internal links exist on desktop but not on the phone layout, they effectively do not exist.
So the work is parity, not responsiveness in the visual sense. Same primary copy, same H1 and H2s, same alt text, same JSON-LD, same meta robots directives, same nav and footer links on both. The classic failures are a stripped-down mobile template, a hamburger menu built entirely in client-side JS, and images that only load after a real user scroll event.
Gotchas: content hidden behind accordions and tabs is fine as long as it is in the HTML, since Google indexes CSS-hidden text. Content fetched only after a tap is not. Also, mobile-first indexing is about which copy gets indexed, not about speed; page experience signals are a separate thing people constantly merge into this one.
Ask AI for it
Check this page for mobile-first indexing parity and fix the gaps. Render it at 412px wide with JavaScript disabled and list every difference from the desktop HTML: missing body copy, dropped headings, absent alt text, JSON-LD that only ships on desktop, nav or footer links that disappear into a JS-only menu, and images behind scroll-triggered lazy loading. Then rewrite the markup so the mobile HTML contains the identical primary content, heading order, structured data, and internal links, using CSS alone to change the layout.