The nature of web pages is a notoriously thorny question. Do you really mean a single HTML document? For most modern web sites the HTML is useless by itself, you also need a ton of auxiliary CSS, javascript, and image files, and usually access to a server for continued service. You can't meaningfully print the page without these.
"What do URLs refer to" was the subject of a long-running argument within the W3C Technical Argument Group that never got resolved and was ultimately dropped (
http://www.w3.org/wiki/HttpRange14Webography). The conversation ranged widely over issues like mutability, format variants, customization (e.g. cookies), and the nature of meaning and reference. The question became the butt of jokes and ultimately I think everyone decided the answer didn't matter, if indeed the question was meaningful at all.
I see that the
schema.org definition of webpage is "a web page". That kind of definition would not be very BFO- or IAO-like. Often a term can be useful even in the absence of a definition, as in this case, and that's fine for
schema.org. But ontologies are supposed to have definitions, i.e. criteria you can apply to a thing to decide whether that thing is a member of the given class.
I would suggest starting, as with other IAO term requests, with use cases. Then induce definitions from there, following the BFO foundation that IAO seems to be committed to, and then decide what the class should be called.