A friend of mine is a web designer, and he says he still meticulously designs to support 3.5" phones and 1024x768 laptop screens, with proper compatibility for larger mobile and laptop/desktop screens of course. (He has worked with really big clients, many of which you would recognize.)
However, he also says a lot of people just don't do this, even though these things are basic things taught in design school.
This makes sense, I suspect because:
1) Some low rent websites aren't designed by web designers.
2) Some people just can't be bothered to test against old hardware. They happen to have an iPhone 6 and a Samsung S5 and if it looks OK on those then job done. Fsck the rest. If you have an iPhone 5, then too bad for you.
3) Some designers just are crappy.
4) Etc.
In fact, this will make a difference in my choice for a new phone this year. My favourite phone size is in the 4.0-4.3" screen size. I find 4.7" too big for my hands, but the problem is that websites are becoming increasingly harder to use on my iPhone 5S.
So, instead of getting the iPhone SE, I will just wait for the iPhone 7. Actually, I'm almost tempted to jump all the way to the iPhone 7 Plus. If I can't use the iPhone 7 easily one-handed, then why not go whole hog and force myself into two-handed mobile surfing all the time?
Another thing that gets my goat in this respect is when a website has been inexplicably optimised for mobile to a point that it's more irritating to browse on a normal computer.
This IMDB page for example:
http://www.imdb.com/scary-good/top-...rd_t=15061&pf_rd_i=homepage&ref_=hm_scg_sf_hd
Yay, scrolling no longer works! Thou shalt swipe, even on a desktop. Clicking the X in the corner results in a sensible rendering of the page that is much easier to browse on a desktop.
Yeah, it seems some websites are now built for 7-10" tablets, so when viewed on a 27" desktop, the text sizes are ginormous. I guess they only want two versions, being the phone version and the tablet/desktop version, except the tablet/desktop version doesn't work well for desktops.