Body element (red border; no width set, implicit width of 100%) + div element (blue boder; 660px width).

The viewport meta tag is NOT set.

The layout viewport width will be set at 980px by default (as 660 < 980).

On a phone with about 400px visual viewport width (e.g. Pixel 2), the scale factor of 0.4 (400/980) will be applied. The page looks smaller.

The page body (red) will occupy the whole screen width and the div (blue) will occupy about two thirds (660/980).

Compare this with 3-660px-meta.html on a mobile device.