My landing page passed every CI check and was still broken on my customer's phone

A customer texted me a screenshot last month. It was my own landing page, open on their Pixel. The headline — "Financial infrastructure to grow your revenue" — was clipped at "...grow your reven". The signup button below it was gray-on-slightly-lighter-gray, basically unreadable. And the hero image? A broken-image icon. Here's the part that stung: every check I had was green. Lighthouse: 98. My Playwright tests: passing. CI: all checkmarks. I had shipped that page an hour earlier feeling good about it. None of my tooling caught any of it. I want to walk through why , because I think a lot of us have this blind spot, and then I'll tell you what I did about it. CI tests the DOM. It does not test what a human sees. This is the core issue. My tests asserted things like "the signup button exists" and "the form has an email input." All true. The button was in the DOM . It just rendered unreadable on a 412px-wide screen with the system in light mode. Lighthouse runs one viewport (usually a throttled Moto G4 emulation) and scores performance/SEO/a11y heuristically . It does not look at your page across the actual range of devices your visitors use and say "this headline is physically clipped on a Pixel 8." And my "responsive testing"? I was dragging the Chrome devtools responsive bar to two breakpoints — 375 and 1440 — eyeballing it, and moving on. That's not testing. That's hoping. The three bugs that slipped through Let me get specific, because the category of each bug is instructive. 1. The clipped headline — a measurable, deterministic bug .hero-title { white-space : nowrap ; /* the culprit */ width : 100% ; overflow : hidden ; } On desktop, the headline fit. On a narrow viewport, white-space: nowrap refused to wrap, overflow: hidden clipped the overflow, and the last word vanished. The brutal thing: this is trivially detectable in code . The element's scrollWidth was greater than its clientWidth . That's a one-line check: const clipped = el . scrollWidth > el . clientW