Hi Katie,
Isn't it ridiculous that when you ask a simply perfectly valid question, all these silly know-it-alls start crawling out of the woodworks and begin to answer you by saying - use the reader, use pinch and zoom, use accessibility... They are so ignorant that they see things in their own narrow vision and refuse to answer the question you are asking.... directly and answer it as they think they know WHAT YOU REALLY MEAN. Why don't they just answer the question that's asked instead of twisting the question and asking why do want to do that. Irrelevant why, but they do!!!
ANYWAY.........
Well, there is a way of doing what you want and it's pretty nifty.
Have a look at this URL http://osxdaily.com/2012/05/11/change-font-size-safari-ios-bookmarklets/
It works by attaching a piece of code to a bookmark. You create 2 bookmarks called "+" and "-". Then you place these bookmarks on your toolbar in Safari.
Then when you view a webpage with the simply awful and crappy default Apple font, you just press the + icon a few times and there you go, the font size increases to a respectable size instead of the unrespectable and horrible default sized midget dwarf microscopic silly Apple font.
I hope this solves your problem.
Maz