When zooming a long webpage that one has already read haflway through makes for a very hard time searching the exact spot that one was when before the zooming started. The problem is that the scroll position is retained, but the page is suddenly much longer, which leads to one jumping up on the page to an arbitrary position. I am sure there is no perfect solution to this problem, but there are several heuristics that can be applied to make it easier for the user to find the location again he was at before he zoomed. * The user could set a marker (select something) before the zooming and that could be retained inside the view. This would probably work very nicely, but requires additional user interaction * On mostly linear pages (like wikipedia articles) the upper limit of the visible document could be fixed so the behaviour is more ore less like it is when you zoom a webpage that is scrolled all to the top - the text just expands and some is moved out of the bottom of the page, but it is much easier to find the reading position again * One could strive to retain a percentage of scrolling - i.e. when the user scrolled a webpage 50%, the scroll position could be restored to 50% after the enlarging. This is a very easy heuristic but will probably work in a high percentage of cases. I personally would like the second alternative best - but I could also live with the third. However the current situation is quite unbearable - especially on larger zoom levels (when zooming two to three times) when the amount you move away from your original reading location can be quite big!
Another possible solution especially when using mouse-gesture based zooming is to keep the point the mouse is over while zooming stable. So you zoom outwards from where you have the mouse when you pinch. That at least would be very consistent with other implementations of this feature and would solve the problem of needing to guess what point to keep stable during the zoom.