Firefox android : Setting -> accessibility -> zoom on all website.

I believe safari by default doesn't respect zoom rules set per website.

and on Chrome Android a workaround is to view as "Desktop site" (toggled in the browser's triple-dot menu) which still makes the text and buttons tiny but hey at least you can pinch-zoom (but unfortunately some panning may be necessary)

Chrome android has the same setting under settings > accessibility

You can zoom in if you open them in new tabs. :}

Workaround: long press, save as photo, zoom in a photo app.