> "Browser" is short of "Web Browser". They're meant primarily for HTML pages.
I don't see how you get from the first of these statements to the second. Web browsers browse the web—an interlinked set of hypertext documents. A "hypertext document" being anything that embeds URLs you can tell the browser to navigate to, usually by clicking. A PDF is a "hypertext document."
> There's no reason to concentrate everything into a single program.
An ePub is also a hypertext document, though. And one that is made out of HTML and CSS files! Would you suggest that a web browser shouldn't be able to open an MHTML archive? Because an ePub is almost exactly the same format, just with a different base CSS style + media selector. (There are some restrictions on scriptability, but from what I recall those are ePub UA restrictions, not restrictions in the standard.)
> Shifting between specialized applications should be seamless.
What if you could shift between specialized applications... inside other applications? Remember OLE? That's how web browsers have traditionally displayed most formats. The whole need for special plugins for QuickTime, Flash, ActiveX, Java, etc. was just to allow those formats to be embedded in HTML pages. But if you're just opening a URL in the web browser and handing the viewport over to a COM component to render—browsers (and many other types of applications) do that just fine, without any need for "concentrating everything into a single program."
I don't see how you get from the first of these statements to the second. Web browsers browse the web—an interlinked set of hypertext documents. A "hypertext document" being anything that embeds URLs you can tell the browser to navigate to, usually by clicking. A PDF is a "hypertext document."
> There's no reason to concentrate everything into a single program.
An ePub is also a hypertext document, though. And one that is made out of HTML and CSS files! Would you suggest that a web browser shouldn't be able to open an MHTML archive? Because an ePub is almost exactly the same format, just with a different base CSS style + media selector. (There are some restrictions on scriptability, but from what I recall those are ePub UA restrictions, not restrictions in the standard.)
> Shifting between specialized applications should be seamless.
What if you could shift between specialized applications... inside other applications? Remember OLE? That's how web browsers have traditionally displayed most formats. The whole need for special plugins for QuickTime, Flash, ActiveX, Java, etc. was just to allow those formats to be embedded in HTML pages. But if you're just opening a URL in the web browser and handing the viewport over to a COM component to render—browsers (and many other types of applications) do that just fine, without any need for "concentrating everything into a single program."