Domains, delivery and craft
What is Mobile-first indexing?
Google indexes and ranks using the mobile version of a page rather than the desktop one. Content, structured data or internal links present on desktop but absent on mobile are, for indexing purposes, absent entirely — which is the failure mode responsive design was meant to end.
Also called: mobile-first crawling
What still goes wrong
- Content collapsed behind a "read more" that is not in the mobile HTML at all
- Structured data emitted only on the desktop template
- Navigation links dropped from the mobile menu to save space
- Images with different alt text, or omitted entirely, on small screens
- A separate m-dot site left partially maintained
How to check it in a minute
Search Console URL Inspection reports which crawler indexed the page and shows the fetched HTML. Compare that against your desktop source. Anything present in one and not the other is the answer to why a page underperforms.
Collapsed content is fine; missing content is not
Content inside an accordion that is present in the HTML is indexed normally. The problem is content loaded only when the accordion opens, which never happens for a crawler. The distinction is in the markup, not in the visual design.
Where this is covered in depth
A definition can only go so far. Technical SEO, in the order the problems actually block you covers this properly — 4 minutes, free, no email required.
Who wrote this
Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan. These definitions come from client work rather than from a content brief — where an entry describes a mistake, it is usually one found on a real site. More about how I work.