marc-eu Hmmm, I'm still not able to reproduce the Page Tweaks font and font size changes not working on the three sample web sites you note above in normal browsing mode.
Are you perhaps using Reader Mode? If so, you can change the reader view's font via the button in the Location Bar (Page Tweaks won't affect the reader view)
Additionally, for the standard web page view, there is an option to set a minimum font size in Settings -> Browsing
