Mobile-first indexing, in Google’s words

Mobile-first indexing means Google uses the mobile version of a site’s content, crawled with its smartphone agent, for indexing and ranking. So whatever is missing from your mobile pages is, as far as Google’s index is concerned, missing from your site; Google says plainly that only the content shown on the mobile site is used for indexing.

General information from Google’s own documentation, not advice for your particular site. Google’s Search Central documentation is the place to check before you change anything, and it is updated over time.

A surveyor’s field book is small enough to carry, and the plan is drawn from what it records. If a measurement never made it into the book, it never makes it onto the plan. Mobile-first indexing works the same way: the phone-sized page is the field book.

Google says a mobile version isn’t required for content to be included in its results, but it is “very strongly recommended”.

Three ways to build for mobile

Mobile configurations Google describes
Configuration	How it works	What mobile-first indexing asks of it
Responsive design	The same HTML on the same URL for every device, laid out differently by screen size. Google recommends it as the easiest to build and maintain.	Content and metadata are already the same on mobile and desktop.
Dynamic serving	The same URL, but different HTML per device, chosen by user-agent sniffing with a `Vary: user-agent` header.	Everything in the checklist below.
Separate URLs	Different HTML on a different URL for each device, often an `m.` subdomain.	The checklist below, plus the extra rules for separate URLs.

If your site is responsive, most of this sheet is already true of it. The rest is for sites that send phones something different.

The parity checklist

Each line pairs what Google asks for with the failure it reports when the two versions disagree. Every line is from Google’s mobile-first best practices.

CheckWhat goes wrong if it fails

Same primary content. Move content into accordions or tabs to save space, rather than removing it.Google warns of some traffic loss when a mobile page carries less, because it can get less information from the page.

Same robots meta tags. No noindex or nofollow on mobile that desktop lacks.Google may fail to crawl and index the page.

No content that loads only on interaction.Google won’t load content that needs a swipe, click or typing to appear.

Resources not blocked. Mobile images, scripts and styles may sit on different URLs.A blocked important image is reported as an error.

Same headings, clear and meaningful on both.Listed among page quality issues.

Same structured data, with URLs updated to the mobile URLs.Reported as missing structured data.

Same title and meta description.Reported as missing title or missing meta description.

Images of good quality, in supported formats, with the same alt text, titles and captions.Reported as low-quality image or missing alt text.

Stable image and video URLs that don’t change on every load.Google can’t process and index the resources properly.

Video easy to find without much scrolling.Google says it might harm the video’s ranking.

Ads that don’t crowd the page, following the Better Ads Standard.Listed among page quality issues.

A detail about images

If desktop and mobile use different image URLs, Google says you may see a temporary loss of image traffic during the transition, because the mobile image URLs are new to its index. Using the same image URLs on both avoids it. Google also notes that it supports SVG images, but can’t index a .jpg placed in the <image> tag inside an inline SVG.

If your mobile site lives at its own address

Sites with separate mobile URLs carry extra duties, because two plans have to stay in step:

Matching error status. If the desktop page works and the mobile page is an error, the page will be missing from the index.
No fragments in mobile URLs. URLs with # fragments mostly aren’t indexable.
One mobile page for each desktop page. If many desktop URLs redirect to one mobile URL, such as the mobile home page, all those pages will be missing from the index.
Both versions verified in Search Console, so you see data and messages for each.
hreflang kept apart. Mobile URLs point to mobile URLs, desktop to desktop.
The desktop URL is always the canonical, with the mobile page as its alternate.
Capacity for more crawling on the mobile host.
robots.txt rules that work for both versions; in most cases, the same rules.

The canonical and alternate pair looks like this on an invented site:

<!-- on https://example.com/ (desktop) -->
<link rel="canonical" href="https://example.com/">
<link rel="alternate" media="only screen and (max-width: 640px)" href="https://m.example.com/">

<!-- on https://m.example.com/ (mobile) -->
<link rel="canonical" href="https://example.com/">

This is Google’s own pattern. The general rules for canonicals are in the canonical URLs sheet.

How to see what Google sees

Google’s troubleshooting advice points to the URL Inspection tool to check that content is visible on the rendered page, which is how Google sees it. Compare that rendering with your desktop page, line by line, using the checklist above.