Yes it's tricky when trying to trying to make sure a book will read optimally on all the different devices you currently own (or may possibly own in the future).
I've found with Kobo less hard-coded css is best, I don't own a Kindle but it seems to have very few on-device adjustments so I guess you have to hard-code more. My Sonys were somewhere in-between and don't get me started on css in the various Android reading apps
