[PaulDavisThe1st]

One reason that this is slightly more complicated than it appears is that you have to decide what ignore when doing the inclusion. Including an image will not override anything specified in the "parent" HTML, but including HTML might result in conflicts. Who wins, who loses, etc. etc.

[harry8]

Why isn't it exactly the same as if the doc had the includes filled out and then processed semantically, much like C?

All <<<include whatever.html>>> could be replaced with contents first. Then it's just a page of html as now.