scale html5 output from FrameMaker 2019?

You could just scale everything to 125% by putting this into your CSS:

html {
    zoom: 1.25;
}

But why would one want to do this? If you need to have things "bigger", why do you not just define them bigger in the CSS in the first place?

Besides that, it does not prevent users from zooming in or out, and some browsers or future versions of them might ignore that anyway.

What's the scenario? In general, with HTML, the scale is controlled by the client device that renders it.

width=100%; is going to be completely different on a 3-inch cell phone screen and a 32-inch desktop window.