> If my browser window is the width of a mobile phone’s screen
It's not, is the thing.
I have a 23 inch screen for my 1080p screen. My phone has a 6ish inch diagonal.
When I make a window 960 x 1080, that's still something like 10 inches across and 11 inches tall. It's sheet-of-paper sized. I'm perfectly comfortable reading that as it was designed for a desktop layout.
I would say that half of my monitor, more accurately, is about the dimensions of an iPad at comfortable viewing distance.
Phones are far taller than half of a 1080p screen.
Half a monitor is: 960 x 1080, or 1:1.125
A iPhone is: 828 x 1792, or 1:2.164
So websites tend to shove a candybar shape into a nearly square sheet of paper and waste a lot of usable proportions for things like controls or hamburger menus.
It's not, is the thing.
I have a 23 inch screen for my 1080p screen. My phone has a 6ish inch diagonal.
When I make a window 960 x 1080, that's still something like 10 inches across and 11 inches tall. It's sheet-of-paper sized. I'm perfectly comfortable reading that as it was designed for a desktop layout.