Alex Jay

That depends on how you want to define it. PDF/Ebook readers or Web annotators allow highlighting and it's not considered "changing" the original material as they are kept separately from the original content. If you refer to the current implementation you are correct, as it doesn't allow highlighting without editing. I'm just pointing out that it need not be that way (as highlighting is implemented without "editing" functionality in other softwares).

It is possible to clip an entire webpage, but to highlight or annotate them it's necessary to convert it to an editable form. This breaks the format and doesn't prevent changes.