The mockup isn't dead. The static one is

I've been starting almost every design direction with a simple HTML/CSS/JS prototype before touching a single design system component.

Not a polished build. Rough code. Fast interactions. Real behavior in a browser.

It's changed three things:

01. Speed goes up when it matters. The polished prototype builds faster because the structure is already solved. No second-guessing layout while picking components.

02. Alignment gets grounded in reality. Product and design stop debating static frames. We're looking at real workflows, real transitions, real component needs. Opinions collapse fast when something moves.

03. You find out what you actually need to build. Edge cases surface. Missing states appear. The prototype doesn't hide gaps, it exposes them before they become expensive surprises in component build-out.

HTML prototypes aren't a shortcut. They're the new source of truth before the source of truth.