That's what I meant by "the promise of the semantic web". As I originally understood it, you could make your own custom tags that describe the intent of the document. You wouldn't be bound by what the w3c or browser vendors blessed. Any tags that were unknown to the browser would be treated mostly the same as a div. Web components occupy a similar space.
> make your own custom tags that describe the intent of the document
If you make up your own tags for everything then to a visitor (human, bot, indexer or otherwise) it describes nothing, since those tags only have that meaning on your site (and worse, they have different meanings on other sites). Semantic web or RDF or Linked Data or whatever you want to call it is not about describing your document within the context of your document, it's about making sure your document is understood in relation to all the documents that link to it and all the documents that it links to.
I guess the semantic web is orthogonal to this discussion. My apologies. I guess it's another buzzword that was thrown around for a time. In that case, assume I meant web components whenever I said semantic web.