MiladiCode/3D-startup-app — reverse-engineered prompt

Build me a modern startup landing page that feels sleek and interactive, using plain HTML and CSS with a Spline 3D model embedded in the hero section. I want it to look like a polished tech product homepage, with a big headline, short intro text, call to action buttons, and a responsive layout that works nicely on desktop and mobile.

The page should have smooth animations, a clean gradient style, and the 3D object should feel like the main visual focus without making the page slow or messy. Please keep the code simple enough for a beginner to understand, and include any basic setup or notes needed so I can open it locally in a browser.

Use Spline for the 3D embed, and if you need the current embed format or best practices, look up the latest docs online.