In 2019, Google completed its transition to mobile-first indexing. This means Google now primarily uses the mobile version of your website — not the desktop version — to determine your rankings. If your site delivers a poor experience on mobile, it ranks poorly in both mobile and desktop search results.
This is a fundamental shift from how many site owners still think about their websites. Building a great desktop experience and then making it "work" on mobile is no longer sufficient. The mobile experience is the primary one, and everything else is secondary.
What Is Mobile-First Indexing?
Mobile-first indexing means Googlebot primarily crawls and indexes the mobile version of your pages. The content Google evaluates for ranking — your text, images, structured data, internal links — is taken from what a mobile user sees, not a desktop user.
If your mobile site has less content than your desktop site — a common issue with older separate mobile sites (m.yourdomain.com) — Google only sees the reduced content. Pages that hide content behind tabs or accordions on mobile may have that content weighted less heavily than content visible by default.
Responsive Design vs Separate Mobile Site
There are three approaches to mobile delivery: responsive design, separate mobile URLs, and dynamic serving. Google recommends responsive design as the simplest and most reliable approach.
Responsive design serves the same HTML to all devices and uses CSS media queries to adapt the layout. One URL, one set of content, no configuration complexity. This is what you should use for any new site.
Separate mobile sites (m.yourdomain.com) require maintaining two separate codebases and careful canonical tag management to ensure Google treats the desktop and mobile versions as equivalent. This approach creates significant technical overhead and risk of duplicate content issues.
Mobile SEO Technical Requirements
Viewport meta tag. Every page must include <meta name="viewport" content="width=device-width, initial-scale=1"> in the head. Without this, mobile browsers render the page at desktop width and scale it down, making text tiny and buttons untappable.
Font size. Text should be at least 16px on mobile. Smaller text forces users to pinch-to-zoom, which Google treats as a negative usability signal.
Tap target sizes. Buttons and links should be at least 48x48 pixels with adequate spacing between them. Tap targets that are too small or too close together cause accidental taps — a key contributor to poor Core Web Vitals INP scores.
No horizontal scrolling. Content must fit within the viewport width without requiring horizontal scrolling. Fixed-width elements — tables, images, code blocks — that extend beyond the screen width are a common culprit.
Avoid intrusive interstitials. Full-screen pop-ups that appear immediately when a user lands on your mobile page are penalised by Google. Small banners, age-verification gates for age-restricted content, and login walls for paywalled content are exempt.
Mobile Page Speed
Mobile connections are typically slower than desktop connections, making page speed even more critical on mobile. Images that are acceptable on desktop can be devastating on mobile. Check your mobile-specific speed scores with our page speed checker — select mobile testing mode for the most relevant data.
Key mobile speed optimisations include serving smaller image variants to mobile devices using the srcset attribute, reducing JavaScript execution time, and enabling browser caching. The image optimisation practices we covered in the previous article are especially impactful on mobile.
Testing Your Mobile SEO
Use Google's Mobile-Friendly Test (search.google.com/test/mobile-friendly) to verify your pages pass Google's mobile usability requirements. Google Search Console also has a Mobile Usability report under Experience that lists any mobile issues across your site.
Check that your SSL certificate is valid on mobile — browsers show security warnings for non-HTTPS pages regardless of device, but mobile users are particularly likely to abandon a site that triggers a security warning.
Summary
Mobile-first indexing makes your mobile experience your primary SEO surface. Use responsive design, implement the viewport meta tag, ensure readable font sizes and adequate tap target sizes, eliminate horizontal scrolling, and prioritise mobile page speed. Test regularly with Google's Mobile-Friendly Test and Search Console's Mobile Usability report to catch and fix issues before they affect your rankings.
Missed the previous article? Read: What Is Schema Markup and How Do You Add It?