Why It Matters
The person searching "tow truck near me" is on a phone. So is the one looking for a restaurant's hours on a Friday night and the one checking whether the groomer takes nervous dogs. For a local business the phone is the first screen, not the second. Google agreed in 2019 and switched to mobile-first indexing: it ranks the site by what its mobile version shows, and a desktop-only site is judged by the shrunken, unreadable mess a phone makes of it.
What A Responsive Page Does
Columns stack. Text stays readable without zooming. Buttons are big enough for a thumb. The menu folds into a button. Images shrink to fit instead of running off the side. The phone number is a tap-to-call link near the top. None of that is a second version of the site; it is the one page, with layout rules for each screen width, built in from the start.
The one line that makes it possible is the viewport tag in the head of the page, which tells the phone to lay the page out at its real width instead of pretending to be a desktop and shrinking everything.
How To Check It
Open the site on a phone. If reading it means zooming, or a button is too small to hit, or the page scrolls sideways, it is not responsive. Google's PageSpeed Insights also reports it, under the mobile tab, along with the load time that usually comes with the same problem.
See it on a working build: every example under Example Sites is built phone-first; open one on a phone and the menu, the hours and the call button are the first screen.
The Foundation Connection
Mobile-first responsiveness is named in the Digital Foundation entry under Performance Metrics. It is not a feature to add. It is how a page is built or it is not, and a site that fails it fails on the screen most customers are holding.
