What web browser features can fix the fact that my 46 year old eyes struggle to read text on my phone or that my fingers (not fat fingers either) sometimes hit the wrong link?
This is a really good point - and all too often designers are too heavily focused on their massive displays to remember that accessibility is very important.
However, this isnt a problem solely focussed on web browsers or solved with apps. If they are going to the trouble of developing an android and iOS app to provide better accessibility, they should have just put a bit more effort in to the front end design in the first place.
Do any browsers support orientation detection? Properly, I mean, not just re-flowing the text. A dedicate app will often adopt an entirely different layout in horizontal mode than vertical mode. Putting buttons along the top instead of down the side for instance.
This isnt the browser - it is down to the design.
Good responsive layouts will identify that the screen dimensions have changed and reformat its CSS appropriately. There are lots of pretty good frameworks to make responsive design trivially easy (320andup is one of the better known) and these all allow for a full reformatting at breakpoints.
Perhaps all of that could be done in a browser if HTML supports it but how many web designers actually do bother to think about that kind of thing?
It does, but not enough developers think about it - they are too busy designing a website for browsers then coding an app for phones.
It's second nature for a phone developer but the most you can really expect a web page to do is flow properly so that page items aren't obscuring other page items.
Phone and web interface developers are, or at least should be, the same beasts.