it is costly for browsers to calculate the current position of each image and cross-reference the scrollbar data, in addition to doing such a thing they would break various image-based tracker mechanisms
I've been a professional C, Perl, PHP and Python developer.
I'm an ex-sysadmin from the late 20th century.
These days I do more Javascript and CSS and whatnot, and promote UX and accessibility.
it is costly for browsers to calculate the current position of each image and cross-reference the scrollbar data, in addition to doing such a thing they would break various image-based tracker mechanisms
That second part sounds like a net win to me.