Web pages are part of the visual presentation of your final implementation, and are not part of any UML Sequence Diagram language construct. Webpages would serve no purpose that is not alredy addressed within the current UML Sequence Diagram language and definition.
Additionally, by including a web page into any design document, you are presupposing a visual implementation. If you are strictly designing sequence logic, you should have NO preconceived notions as to how the visual implementation might look. (It's not a "website design diagram", it's a sequence-of-logic diagram.)
I find that you can almost always make a case to violate any guiding principle, design approach, teaming agreement, programming language paradigm, and even engineering "best practice". But these are (and should be) "red flags" that alert you to the fact that you are going outside of the original concept/reason for using whatever technique/tool/etc. you've chosen, and it will only lead to less rigor. Rigor, for its own sake, is not good or even necessary. But rigor constraining behavior for a specific goal is usually "A Good Thing(tm)", because it saves you from yourself - or from "misuing" (of sorts) the framework that you've selected.
Suggestions/frameworks/best practices exist solely to advise and constrain your behavior in favor of a particular outcome, and are based upon the combined experience of many, many very talented people. I don't recommend that you violate the UML Sequence Diagram construct by including webpages in the diagram, as this violates the rigor of the UML Sequence Diagram constructs, and it defeats some of its purpose (to remove visual - and other - considerations from what should be rather pure business logic and/or the interactions of your design).
I recommend that you stick to using the UML Sequence Diagram a little more literally - it's not a "website design diagram", unless THAT is what you WANT to make it. And if so, you might call what you are making a "website sequence diagram", though it seems to be a stretch...
It's up to you to use your tools as they suit your needs. But I wouldn't make this change lightly. =)