I am watching the Rapidweaver SEO course at the moment, which I bought because I am trying to get more clients to my business websites, and I know very little about SEO.
I have just watched the part about SEO friendly HTML. One thing that I learned was to check a website without any CSS, to view the HTML structure. I checked both my websites, www.marklukens.nl and www.indetailfotografie.nl. On both websites I used the “Hide my stack” feature a lot to to make it look nice on desktops, tablets and mobile devices. However when looking at the raw HTML, it looks messy to me because a lot of content is displayed twice or even three times. Is this something that could hurt my ranking, as it could be seen as spamming for example?
The short answer is yes it probably does hurt your SERP ranking. Probably not because of spamming but as duplicate content, plus hidden content still gets loaded (as you have seen) for each device. That increases your weight of each page, slowing your load time. Google and all the search engines are using page weight (load time) more and more than they used to. Google is now really pushing the AMP results, mostly to speed up load time for mobile users.
I agree with Doug but to what degree it will hurt… I think little. That being said, on stacks you can use Joe Workmans Agent Stack to accomplish what you are after. Agent will not load all the content, just the content for the device viewing the page.