Mobile-first indexing

Mobile-first indexing means Google uses the mobile version of a site’s content, crawled with a smartphone agent, for indexing and ranking. Put simply, Google judges a page by what a phone user sees.

What is mobile-first indexing

Google started mobile-first crawling and indexing in 2016. On October 31, 2023, Google announced the switch was complete; only a small set of sites that do not work on mobile are still crawled by the desktop Googlebot (Google Search Central Blog, Mobile-first indexing has landed, accessed November 2026).

Content that exists only on the desktop version is effectively invisible to Google. Google says a mobile page with less content can lose some traffic, and suggests moving content into accordions or tabs instead of cutting it (Google Search Central, Mobile-first Indexing Best Practices, accessed November 2026).

The viewport tag: what the AIO checker scores

The viewport meta tag tells mobile browsers to fit the page to the screen width. The standard tag goes in the <head>:

<meta name="viewport" content="width=device-width, initial-scale=1">

width=device-width matches the page to the screen width. initial-scale=1 sets a 1:1 ratio between CSS pixels and device-independent pixels. Without the tag, mobile browsers lay out the page at a desktop width, usually about 980px, then shrink it (web.dev, Responsive web design basics, accessed November 2026).

Hypothetical example: a 390px-wide phone showing a page laid out at 980px shrinks everything to about 40% (390 / 980). 16px text becomes roughly 6px, and users have to zoom in to read it.

Adtek’s AIO checker gives the “Mobile viewport” check 2 points and passes it when the page has a meta viewport tag.

Mobile-first indexing: what must match on both versions

Google recommends responsive web design (one URL, a layout that adapts) because it is the easiest pattern to implement and maintain. Sites with a separate mobile version (for example m.example.com) need a closer check (same Best Practices source):

• The same main content as desktop, including text, images and video.
• The same structured data.
• Equivalent title elements and meta descriptions.
• The same image alt text and the same robots meta tags.
• Main content that does not load only after a tap or swipe, and does not appear only after JavaScript runs (see JavaScript SEO).

Why it matters for AI search

To be shown as a supporting link in AI Overviews and AI Mode, a page must be indexed and eligible to appear in Google Search with a snippet; Google says there are no additional technical requirements (Google Search Central, AI features and your website, accessed November 2026). Since Google indexes the mobile version, content missing from mobile is also not available to Google’s AI features. For ChatGPT or Perplexity crawlers, no company has said the viewport is a signal.

How to add the viewport tag when the checker flags it

1. Check. Press Ctrl+U to view the page source and search for “viewport”.
2. WordPress with a block theme (edited in the Site Editor): WordPress has printed the viewport tag itself since version 5.8 through the function _block_template_viewport_meta_tag (WordPress Developer Resources, accessed November 2026).
3. WordPress with a classic theme. Modern themes usually include the tag in header.php. If it is missing, use the theme’s header code field (if it has one), or create a child theme, copy the parent’s header.php into it and add the tag inside <head>, so parent theme updates do not wipe it.
4. Shopify. The <head> lives in the theme.liquid layout file (Shopify Dev, Layouts, accessed November 2026). Popular themes usually include the tag already; for a customized theme, add it there.
5. Custom-built sites. Add the tag to the shared template used by every page.
6. Check again. Run Lighthouse in Chrome DevTools: the audit “Has a <meta name=”viewport”> tag with width or initial-scale” should pass; as of Lighthouse 13 it sits inside the “Optimize viewport for mobile” insight (Chrome for Developers, Lighthouse viewport audit, accessed November 2026). Turn on DevTools device mode to view the page at phone size. URL Inspection in Google Search Console shows whether the page was crawled with a smartphone or desktop agent. Finally, run the AIO checker again.

Do not look for the Mobile-Friendly Test. Google retired it, along with the Mobile Usability report, starting December 1, 2023, and pointed to Lighthouse instead (Google Search Central Blog, Page experience in Search, accessed November 2026).

Frequently asked questions

Will a site without a mobile version be dropped from Google?

No. Google says a mobile version is not required for your content to be included in results. Phone users will just see a shrunken, hard-to-read page.

Is mobile-first design the same as mobile-first indexing?

No. Mobile-first design usually means designing for the phone screen first. Mobile-first indexing is how Google indexes pages. Sites designed mobile first tend to have fewer issues when Google reads the mobile version.

If you need a review of your site’s technical foundation for Google and AI search, see Adtek’s SEO solution.