Consider following markup:

   <div>
     some text
     <foo>first</foo><bar>second</bar>
   </div>

There is no way in CSS at the moment to define content flow of the div above. The only possible way is to define `display` on its children.

Say, you want that div to replace content vertically as if all children are blocks and text runs are wrapped into anonymous paragraphs:

   <div>
     <p.anonymous>some text</p>
     <block.foo>first</block>
     <block.bar>second</block>
   </div>

That's impossible in modern CSS without redefining `display` in children. But usually you cannot blindly say

   div > * { display:block;} 

What if some child uses `display:flex` ?

Problem is that `display` defines how element is replaced among its siblings. But not how its children are laid out.

At some point it was a proposal to add `display-model` css property. So you can have

   div { display: block; display-model:flex | table | etc }

Without it we have ugly semi-solution of combinatorial explosion:

   div { display: flex; }
   div { display: inline-flex; }
   div { display: table }
   div { display: inline-table; }
   div { display: grid; }
   div { display: inline-grid; }
   ...

Ah OK, I agree this would be useful. I think a newer CSS proposal factors the display property into separate values to declare element flow and element content flow independently.

"a newer CSS proposal"

This is of 2002: http://www.w3.org/TR/2002/WD-css3-box-20021024/#L706

15 years ago, Carl!

Yeah, that draft was abandoned though, but some of the ideas have been incorporated in the CSS3 display model.