title_body: CSS: Spans vs Font Families
upvoted_answer: Before looking at anything, you might want to look on the available fonts for each device. What are serif and sans serif fonts on different ebook reading systems? It doesn't show iBooks as having Arial or Verdana fonts, so it will be unable to render these fonts. When you edit it in Sigil, chances are that the computer browser already has Arial and Verdana fonts loaded, so that is why it looks different on your computer vs. your device.  
  
  To be honest, I am perfectly fine with using  "font-family: sans-serif;" or "font-family: serif;" .iBooks and Kindle both have excellent fonts, and users often like to specify a favorite.  It can be maddening trying to specify fonts for each device unless you're embedded fonts for ALL devices. (This guide will get you started: https://github.com/JayPanoz/Soma/wiki/How-to-embed-fonts and here's a listing of some google font combinations which you could use for ebooks if you decided to embed the font: http://fontpair.co/  ).
  
  I should stress again that embedding fonts is a complicated deal; I've started trying it myself with my ebooks. I am very taken with Google's Merriweather font for the body text.