Hello everyone 
Thank you for all your comments.
I’ve been away for a few days … travelling.
After reading @duncan’s comment, I think I have caused some confusion with my term “mobile-first”. I did not use it correctly. Sorry.
When I mentioned “mobile-first”, I was merely thinking from the web-design-point-of-view and not the output of the actual code.
Web design with Sparkle is different and you have to do a bit of rethinking and reimagining, especially when you switch from a less “visual” website builder. But once you know and understand how Sparkle works, I think it’s actually quite easy and fun.
When I built my first website with Sparkle, I made the following mistake:
I started with a new file and created all layouts for all devices right in the beginning. And then while designing the website, I was constantly adjusting each layout. Yes. I know. That was a stupid idea. I found out the hard way. 
So now I start with one layout, usually desktop (960 pixels). Once that is finished, I do the 320 pixels layout.
And when that is completed, I just add the other layouts (768 pixels and 480 pixels). Since those layouts are created from the other two layouts, almost no additional adjusting is needed – just minor changes. That workflow can be quite quick.
When I do the first layout (960 pixels) I find it quite helpful to make my own “sections” with groups of elements (boxes, text boxes, images …) because then I can move those groups around in the other layouts and adjusting is much easier.
So, I learned from my mistakes. Knowing how Sparkle works, I take its features into consideration when planning a new website.
So, my post was about starting with the 320 pixels layout instead of the 960 pixels layout and working the other way around with the design.
But I’m glad @duncan provided so much background information, it helped me a lot in understanding what “mobile-first” really means. So, thank you @duncan!
I am sure that Sparkle produces great code for each device so we no longer have to worry about all that. 
We can concentrate on the design.
And that is the beauty of Sparkle … I am so looking forward to new features in the future and to see where our journey with Sparkle is going.