If you just want to annotate a web page without assistance from that page,
and let you share then annotations, that's not a Wikimedia-specific problem
and there are generic tools for it. The popular one is Hypothesis
<https://web.hypothes.is/about/>, I think. There even was an attempt to
build something Wikimedia-specific on top of it (see the task Kosta
linked), but it can be used directly as well.