Hey Brian,
usually HTML pages should be designed to accomodate any screen width and I tried to resize my (remote) web browser to see if this one does and it allowed zooming out, but in the other direction and as you also show here, if the native width of the monitor is smaller than a specific fixed size (or if you zoom in too much), it ceases to accomodate to width and you have to scroll horizontally, which is not ideal either.
Anyway your photo makes clear what advantage the portrait orientation gives here.
What is the native resolution of this monitor?