I love stuff like this that talks about web basics in precise detail. Too much of the stuff out there for beginners glosses over the basics of the web. In the midst of receiving tons of info about HTML & CSS & Javascript & whatever scripting language beginning devs have trouble knowing at all times what level of the stack they are operating at, what's the execution context. Can I write Javascript here? Can I write Ruby/Python? Does it have to be interpolated? This stuff is seemingly simple but is actually so complicated when you're thrown into the ocean that beginners have trouble keeping track of it.
And it really doesn't help when more experienced devs, or non-web programmers shame people about web dev being dead simple because it's not. It's a different kind of programming, it may even be easier than writing C and managing memory explicitly and not having garbage collection. But if it is easier, it's only easier once you've mastered it and have a really strong grasp of what is happening at all times. Until you get there, it's easy to get lost in the endless list of technologies you must be familiar with to write even a simple web application. And I know this because I see it in beginners that I have taught and/or worked with.
I recently decided to write from some tutorials[1] (maybe eventually a book) that assumes zero knowledge and works its way up. I'd like to eventually go all the way up through SQL, basic httpd administation, etc. It's a long row to hoe but I think it may be valuable.
I would strongly recommend reviewing your 101 WRT what you're calling tags and clarifying that tags represent elements that are interpreted and rendered by the browser. (The browser does not render tags.)
Also, the distinction of semantics and styling should be cleared up a little. Less emphasis on how elements are styled by default, and more on what they represent.
I like the approach of assuming no knowledge, but this requires extra care to introduce concepts without using improper or misleading terminology. As a beginner, there's not much worse than hearing simplifications and thinking they're the whole story.
Both your site and the original post are great starts.
Anyone reading or writing these sorts of fundamental web programming tutorials should look at Philip Greenspun's books Philip and Alex's Guide to Web Publishing and Software Engineering for Internet Applications, both available free online (The latter is a stripped down version of the former, written for goal-oriented MIT students who don't want anecdotes or pretty pictures). They are around a decade old, so they aren't a good source for copy-paste-ready code, but the ideas in the books have aged quite well (from simple things like using "abstract URLs"--URLs without extensions like .aspx and .php, to focusing on the website's data model and user interaction instead of the programming language).
The most interesting part of these books is that they explored creating web applications for a purpose, not as an end in themselves. They were written for the sort of people who evaluated their past year not on what their manager wrote in some HR form and the raise they got, but on what they were able to accomplish, like creating http://scorecard.goodguide.com/. (What's funny is that in Greenspun's ITConversations interview (around 4:20 http://web.archive.org/web/20130729213414id_/http://itc.conv...), he said that the parts of Phil and Alex's Guide that aged were not the technology, but the rampant idealism in the book that web applications would unleash a new age of enlightenment, like the academics who had believed television would be used to broadcast Harvard lectures around the world).
(It just occurred to me that Philip's interview above is one day short of being 10 years old, and his assessment might be worth revisiting. The internet still contains volumes of crap, but the internet as only grown as a teaching and community building tool, with sites like Meetup and Facebook organizing learners and teachers, Khan Academy and Udacity doing organized classes, and StackOverflow collaboratively solving specific problems).
And it really doesn't help when more experienced devs, or non-web programmers shame people about web dev being dead simple because it's not. It's a different kind of programming, it may even be easier than writing C and managing memory explicitly and not having garbage collection. But if it is easier, it's only easier once you've mastered it and have a really strong grasp of what is happening at all times. Until you get there, it's easy to get lost in the endless list of technologies you must be familiar with to write even a simple web application. And I know this because I see it in beginners that I have taught and/or worked with.