Common failure modes: a hero image that fills the entire mobile fold (no CTA visible), a navigation menu that pushes the headline below the fold, a CTA button that is wider than 100% of the viewport (visible but unusable), text-on-image hero copy that becomes illegible at small sizes.
The fix is not "make it responsive" — most pages already use responsive frameworks. The fix is designing for the mobile fold first. If the four above-fold elements (headline, sub, CTA, proof) all fit in 600 vertical pixels on mobile, they almost certainly work on desktop too.
Mobile load time matters as much as layout. A page that takes more than 3 seconds to render on a mid-tier phone loses visitors before they see the content; the share lost climbs with each additional second. Compression, font subsetting, and deferring non-critical scripts are the cheapest wins.
How Lytms scores it
Lytms scores mobile layout under the Post-click dimension. The mobile-breakage module renders the page at 390px wide, checks whether the four fold elements are visible, and surfaces specific elements that overflow or get clipped. The perf-blocker module catches load-time issues that show up disproportionately on mobile.
See also
From the blog
See mobile layout on your own page.
One URL. About 2 minutes.