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Ugh, no. Layout is one way to express those relationships, but those relationships already exist in the HTML. This is important because different layouts can express the same relationship. I can put a caption below, above, beside, or over a photo and have viewers understand what is meant but which option is appropriate can depend on a number of factors that have nothing to do with the actual content or relationship being expressed. Blind people read the Web too. Having multiple layouts for different kinds and sizes of device is super useful. Being able to attach a caption to photo in HTML and express the exact spatial relationship later through CSS is much more versatile than hard baking layout into the document.

" You would need to express the semantic relationships within the data, like "comment-replied-to" and "photo-with-caption" and so on. That sounds tantamount to a full implication of natural language processing."

No you wouldn't and no it is not. It's dead simple:

    <figure>
        <img src='image.jpg' alt='missing' />
        <figcaption>Caption goes here</figcaption>
    </figure>

Sure, it's dead simple for one particular semantic case already with its own specific definition in HTML. But you cherry-picked that from my examples. HTML doesn't express semantics for comment-replies or sports scores or so on. It has a generic notion of a hierarchical tree, to which CSS can apply layout (like indentation for HN comments), but that's not quite the same thing. Nothing in the HTML-CSS stack expresses general-purpose semantic relationships and we tend to make an inner-system-effect mess of it when we try.

I'm going to cherry-pick your examples again and point out that sports scores actually an ideal use case for actual tables because they are generally actual tabular data. But frankly I just disagree when you say that a hierarchical tree isn't a mechanism for expressing general-purpose semantic relationships, when it's at least as good as spatial positioning on a page. In fact, I can use classes with semantic names which make my meaning perfectly clear to anyone reading the source. It might be invisible with a bad or default stylesheet, but that's a relatively easily resolved problem.

> Right here on HN is an example, where the comment reply chain is indicated with indentation.

    <div class=comment>
      <span class=user>T-hawk</span>
      Right here on HN is an example, where the comment reply chain is indicated with indentation.
      <div class=replies>
        <div class=comment>
          <span class=user>random28345</span>
          HTML is a tree of element nodes, the identity of the post being replied to can be determined by 
          the position of element in the tree 
          <div class=replies />
        </div>  
      </div>  
    </div>

----------------------

    .replies {
      margin-left: 2em;
    }

Exactly what I thought. The CSS make a visual indentation, but carries no more semantic than the data structure does already.

In fact, if it did, I would consider the HTML broken.

    <div class=comment.level1>
    ...
    </div>
    <div class=comment.level2>
    ...
    </div

> You would need to express the semantic relationships within the data, like "comment-replied-to"

Like https://indieweb.org/in-reply-to ?