Snagit

Thanks to James for recommending the screen capture app Snagit, which is now in beta for the Mac (existed for years on Windoze). I'm trying it, and it's excellent. It can auto-scroll and capture a tall web page in one shot. And there are various and very easy ways of putting on comments on a screen capture, and formats to save it in, including PDF, where the added text is selectable.

(This is scaled down by Blogger, but you can download a 11MB file here, with both the PDF and the JPG versions in full size.)