title_body: What are serif and sans serif fonts on different ebook reading systems? In epub and kindle css, you can specify a fallback font by specifying 
  
   p {font-family: "Georgia", serif}
   h1 {font-family: "Helvetica", sans-serif}
  
  
  If various reading systems have defaults for serif and sans-serif, can you tell me what they are? 
  
  I.E. Kindle e-ink, Kindle android app, Google Play Books, ibooks, Kobo, Adobe DE. 
  
  On a related question, can I assume that all titles are in the default sans-serif font? When a customer overrides publisher defaults and chooses a certain font (like Bookerly on kindle), does that leave the title fonts still in the default sans-serif? 
  
  UPDATE: Someone on KDP forum has confirmed that user-selected fonts for kindle systems are the same for body fonts and heading font. Therefore, you would need to specify a heading font if you want it to be different. 
  
  UPDATE 2 I encourage people to post fonts existing on  the mobile devices or reading systems that they have. Please indicate whether there is an option to specify Publisher Defaults on the reading system. Also, if you have any insight as to which font would be considered the default Serif or Sans Serif choice by the reading system, please mention it.  Thanks! 
  
  UPDATE 3 (5/2017) A commenter has pointed out that some Amazon  Kindle devices are using  fonts for Indic languages written in Bengali/Devanagari/Gujarati/Kannada/Malayalam/Tamil/Telugu. On the device this font is referred to as 
  "Noto Sans."  Noto is a multilingual font developed and supported by Google, but Amazon has only used  portions of this Noto   font for  Indian languages.  I have seen no official announcement yet.
upvoted_answer: Kobo ereader devices
  
  My 1st gen Kobo Glo has the following fonts pre-installed (I suppose other Kobo devices use the same set; it is possible for the user to add more but I think this is beyond the scope of your question):
  
  Serif: (1)
  
  
  Amasis
  Caecilia
  Georgia (Default Serif font; also default font if nothing is specified in the ebook)
  Kobo Nickel
  Malabar
  
  
  Sans Serif:
  
  
  Avenir Next (Default Sans Serif font)
  Gill Sans
  
  
  Other:
  
  
  KBJ-TsukuMin Pr6N RB (Serif) (2)
  KBJ-UDKakugo Pr6N M (Sans Serif) (2)
  OpenDyslexic
  
  
  Of course, these customizations are optional, and it is possible to default fonts as defined inside the ebook.  
  
  About titles, if there are not specific settings inside the ebook stylesheets, they are treated like other text: besides having a larger size, if not explicitly set to something else, the default Georgia is used.
  
  Kobo app for Android
  
  Compared to the ereaders, the app is more limited in the customization options:
  
  
  Default settings
  Droid Serif
  Droid Sans
  
  
  
  
  1. Including slab serif
  2. I'm not really sure but I think that these are used for Chinese/Japanese languages compatibility.