Websites' default font and size for readable text content can be set under the css for body and then not overridden in more specific tags or classes, so the UA

Websites' default font and size for readable text content can be set under the css for body and then not overridden in more specific tags or classes, so the UA can easily override that without breaking all other areas that need custom fonts and sizes like navigation. Users not knowing how to change their font or font size is not an excuse for website designers to choose fonts and font sizes whimsically, since they rarely base their decisions on good quality readability data. Browsers and other UAs need to step up and provide meaningful interfaces to choose those defaults at install time, and make it easy (and easily discoverable) to adjust.