Linking and other non-linear style:
How to link from one page to another. (Using that in a way that goes beyond “Page 1″, “Page 2″, …) This can both be anchor tags in html, but also things like embedding content in video, or any other outbound-pointer.
Why? Participating on the web means connecting your content with other content. This is both useful as a form of citation, but also in constructing a non-linear narrative.
Why not? It’s not essential to have links in order to create content online.
Style/Formatting (tags, etc.). ”Restaurant HTML”:
A basic knowledge of how to style content. For example, big bold text, small text, colors, etc. Laying out a page how you want it to look.
Why? I don’t have a good “why” grasp here, but I intuitively feel like this is important. If all of your pages are totally plain-text, you can still get your message out, but it definitely loses some of the webiness, doesn’t it? I feel like it’s the social equivalent of poor spelling. Hrm.
Why not? If all of your pages are totally plain-text, you can still get your message out.
Content vs Theme:
An understanding of the difference between the content and the theme. Does not necessarily imply knowing CSS or style tags, but rather a grasp of how the style of something can change but the content remain the same, and vise-versa. The difference between the two.
Why? It’s been my experience that learners get excited about skinning the web (leaving the content alone) to their own aesthetic taste. Understanding the difference between the two is the first step here. It’s also an important step towards asking the right questions in web making: “where should I put this? how do I want this to look? what should this do?”
Why not? How is this different from offline media? Newspapers also have content and theme. Is this really a web-maker skill, or just a life skill?
Remixing:
Taking parts of the existing web, and changing them.
Why? Again, having trouble putting my finger exactly on why beyond a feeling here, but it seems like this type of building-on-something-else has become one of the strengths of the web.
Why not? 1) You don’t have to remix to web make. 2) This comes with a lot of necessary “safety” skills that it’s irresponsible to not teach, about copyright laws and the like. So it’s a heavyweight topic.

Specifically on the “style/formatting” point, I guess the “why” would be “effective communication”. If you have an idea and you write it down in plain text, in a mono-spaced font, in one long line, as a single run-on sentence, I guess you’ve “created content” but nobody will want to read it.
Part of effective content creation is very definitely the ability to take some big idea, break it down into small chunks, then communicate how those chunks can be put together to form the larger idea. The most basic tools are sentences and paragraph breaks, but headings and subsections and sidebars and ordered/unordered lists and footers are all in the toolkit. Understanding when each of those tools is appropriate, and knowing how to apply them, is something content creators need to learn.
Linking: The why not – no, linking is not essential to having content online, but without it is there really a reason to put your content online? It’s very hard for anyone to find anything on the web without linking, how would people find this unlinked content? Especially since if you can’t understand linking, you won’t understand metadata either. I’d call this one essential.
Style/Formatting: The why – it’s not about losing the webbiness, it’s about retaining information and being able to process it. The theory of visual learning will support your suspicions, this is important!
Content vs Theme: I agree with your why not. This seems like a life skill to me, and I think it’s one that every child learns pretty early on (Mommy is still mommy even though she’s wearing a dress instead of pants).
Remixing: The why not – yep, heavy topic with a lot of side skills that have to be understood, but this is what makes the web great. I’d call it important.