A site that looks correct on one designer's laptop can break in ways that cost conversions on a different browser or a mid-range Android phone, which is why cross-device testing is run as a dedicated pass rather than an assumption. We test across real browser and device combinations using BrowserStack, supplemented by automated testing with Cypress or Playwright for regression coverage on repeat deployments.
This applies to every build we deliver, but gets particular attention on e-commerce checkout flows and lead-capture forms, where a broken field or unresponsive layout has a direct, measurable cost.