I'd guess unrelated to the web page you were using - either malware or some benign app doing something weird. If you remember the exact date and time you could have a look in Console app to see if there's anything relevant in the system logs.
Are you 100% sure it was Safari and not something with a similar window/icon?
The webpage I was browsing in Firefox at the time was the Grauniad and as mentioned, Firefox is the default browser, by choice and setting.
The situation occurred about 10 minutes before I posted on here. Never used or needed to use Console before
Certain it was Safari, in that the blank window appeared and then immediately hid itself behind the Firefox window, before I could identify it, (no other windows or Applications were running), so I checked the topmost 'toolbar'? and it showed Safari as the active application
I then checked the Dock and it confirmed Safari was running. As I was 'startled' and about to try and quit Safari, it 'quit' itself with no input from me and no keyboard or mouse use. It basically appeared and then disappeared in a matter of seconds.
Feel more relaxed now - 27 mile ride in cool drizzle - but NO gales and NO flooded roads - plus tonight's supper ingredients procured