I've not recently tried any CSS updates, but I have recently had an interesting revelation. A year or so back I tossed together a simple HTML file with a table containing names of an assortment of my local files on this desktop, mostly Excel spreadsheets, that I use to track some stuff and update frequently. It loads from the home screen in my browser (Firefox on Win7x64 here) and allows me to just click on "Rainfall" and the spreadsheet I track the garden rain gauge with opens (separately, not in the browser). Suddenly in the last week or two, when I do that and then try to save the file, I get flagged that the "file is read only" and asked to choose another name. What I found is when that file is opened by Firefox, it is apparently copied to the temporary application file area and set to Read Only. An attempt to save the file is trying to place it back in the App data area.
I suppose in some ways, this is good in terms of security, but it was a radical and sudden change in behavior.** I've been assuming the change was a consequence of a Firefox update (I'm currently at 49.0.1), but there have also been the usual monthly piles of Windoze updates along the way. I've no idea whether FF directly plays with those files at opening, or whether it's using some Win system calls, in which case the change could have occurred there.
Anyway, I've no idea whether that could have any connection with the behavior described in the OP, but it shows how we can never let our guard down!
DaveT
** As when I relented and updated my iPhone 5c to iOS 10.xxx and the most fundamental function -- pressing the home button and swiping the lock screen does something different from iOS 7, 8, and 9!