Signal 03 of 09 · Nebula Diagnostic Spec v1.0.0

Mobile CTA

Is the primary action visible and usable on a small viewport?

Decision rule

The primary call-to-action must be fully visible inside the initial 390px-wide viewport without scrolling, and must meet minimum tap-target size. A run flags CTAs pushed below the fold, overlapped by other elements, or smaller than usable touch targets at mobile width.

What the engine inspects

  • ·CTA bounding box position and size at 390px
  • ·overlap checks against header, banners, and sticky elements
  • ·tap target dimensions

How to check it yourself

  1. Open the page at 390px viewport width (DevTools device toolbar).
  2. Confirm the primary CTA sits fully inside the first viewport with no overlap from headers, banners, or sticky bars.
  3. Check the tap target is comfortably large; 44px minimum height is the working floor.

The engine automates this inspection and attaches measured evidence to every failing condition. A manual pass is not a substitute for the recorded evidence trail, but it should agree with it.

Pass / fail examples

Pass

Primary button renders entirely inside the first mobile viewport with a 48px tap height and nothing covering it. Clear.

Fail

At 390px the CTA sits underneath a fixed bottom banner, so the first tap hits the banner instead. Flagged.