为什么在块级和内联元素之间HTML在功能上存在差异？

Question

Note: I am NOT talking about the display - I understand why they have different defaults. I'm talking about the functional differences between blocks and inline elements, esp. in regards to things like nesting

I've been told time and again that HTML is about describing content, what things do and what things are, not what they look like (beyond default stylistic elements associated with tags, that can be changed).

As such, it seems like the division between "block"-level tags and "inline"-level tags (not the display of those tags, but the inherent difference in those tags), something that trips up and complicated a great many neophyte and even more experience page designers, is arbitrary and... seemingly against principle, if you would. This sort of behaviour seems like something that should by controlled stylistically, not exist as a built-in element of a block. And in some ways, it IS controlled stylistically - we all know there is CSS that can make a span look like a div, and vice-versa.

So what is the point of having the functional distinction between blocks and inlines? Why aren't they just like any other nested element? Why isn't everything a "block" with "inline" elements simply set to "display: inline"? Is it just for, say, backwards compatibility, or is there a benefit to this seemingly arbitrary division that I'm just not seeing?

Note, this might be better as a community wiki question, since it's unlikely to have a single clear answer, but I'm not exactly sure on the details.

As an example, from what I understand, the following would be invalid html:

<label style="display:block">
 <h4>Name</h4>
 <span class='sub'>Whether legal, pseudonymous, or made up</span>
 <input id="name"></input>
</label>

Why?

Answer 1

Why isn't everything a "block" with "inline" elements simply set to "display: inline"?

Well, it almost kind of is; just the inverse. Rather, everything is "inline" with "block" elements set to "display: block".

But that would be really annoying, if for every page you create, you need to tell it that div tags should be treated as block elements. So for that reason, default styles are assumed, best illustrated at http://www.w3.org/TR/CSS2/sample.html