I use Firefox on my laptop. I have it set to override the website's choice of font with my own default font. Usually I choose Arial Rounded MT Bold as my default font, except when it's important for me to distinguish between boldface and non-boldface - for example, when I'm using HTML code in one of my own posts.

For mobile Safari, all I can do when I'm having trouble reading a page is switch to the Reader view in Safari, which provides me with a slightly larger, boldface font. When my eyes get really bad, I abandon Safari and switch over to the Perfect Browser app, which allows me to specify font style and size.

Unfortunately, none of this is obvious stuff. It took me a while to figure out there was a Firefox setting that would let me routinely override the website's fonts. And I had to spend hours and hours on the Apple app store, searching for an iOS mobile browser that would let me read in my choice of font. So I think it's fair to assume that some of your website's visitors are simply going to accept whatever font you choose for them, because they don't have the knowledge or the software to choose for themselves.

But if they do know how to override your font choice, that should be easy enough for them to do, at least in the browsers I've used. I don't think you need to do anything special on your end.

Though maybe jesse_the_k has more to say on this topic!