Do the HTML font tags (like p and h3) have default font sizes universally in all browsers? For example, I'd imagine p is around 16px or so. Is there a standardized list someone could link me off to?
Browsers can implement any size they want to and usually do. That's why you should always implement a CSS reset of your own. This way you are sure all browsers use the same default styles for all HTML elements. There are a lot to choose from but if you look around you can see they all follow the same basic principles.
user-agents defaults typically differ, from little subtleties, to horrible travesties (trident :) ). these sites cover the vast amount of the most popular ua's today:
older ua defaults: http://css-class.com/test/css/defaults/UA-style-sheet-defaults.htm