When creating digital media based on web technology, e.g. HTML5 websites, web apps or ePub3, and which do not have any webfonts embedded or linked, then you rely on the fonts that are pre-installed on devices.
Apple’s iPad (and iPhone) actually have quite an impressive list of fonts pre-installed, for example
- Avenir (in many faces)
- Gill Sans
- Snell Roundhand
- Times New Roman
- and others
The website iosfonts.com lists all of them and also shows you which version of iOS added them on the iPhone and which on the iPad.