The majority of web traffic is mobile, and Google indexes the mobile version of your site first. Designing for the small screen first isn't a constraint — it's clarity.
Design the phone first
Start with the smallest screen and the single most important action. If it works there, scaling up is easy; the reverse rarely is.
Make touch targets generous
Buttons and links need room for thumbs. Tiny tap targets and dense menus are the fastest way to lose a mobile visitor.
Cut weight ruthlessly
Mobile networks are slower and patience is shorter. Trim images, scripts, and anything that doesn't earn its place.
The takeaway
Design for the thumb, then enhance for the mouse. Mobile-first is just user-first.