We are considering upgrading from RH 2015 to RH 2017, and I have started our trial period. It looks to me like it stresses HTML5, but we've been using Web Help for our output SSL. I see a couple of advantages to 2017, such as installing entire folders into the baggage files folder and having the option to use thumbnails for images, etc., but are there really any significant advantages to upgrading?
Also, I've noticed that the .css files sit in the Project Files folder along with the topics. How do I import a .css file?
You say "it stresses HTML 5" and I'm assuming you are meaning that Responsive HTML5 is the preferred output. And you would be correct. That is where ALL the development has been happening. Indeed, adding folders of files to Baggage is one of the awesome features I've longed for and has finally happened. And thumbnails are nice. But there are other nice things too. For example, predictive search. The search now behaves almost like an index, where as you type it tries to match words in the system. But I'm thinking that this only applies to that Responsive HTML 5 output too.
You import a CSS file via Topic Properties. Just look at the properties of nearly any topic, then click the Appearance tab and note the little folder icon. You use this icon to browse to and choose a CSS file you want to bring in.