Top of Safari browser window blushing red with El Capitan

Two days ago I updated to El Capitan (10.11.3), running it on my iMac (mid 2011; 2.5 GHz Intel Core i5; 4 GB 1333 MHz DDR3).

Since then, when using Safari (9.0.3) and browsing different sites, the top inch of my browser window has blushed red (and these are not blush-worthy sites!). After a few minutes at a site (these are completely random; I discern no pattern), a reddish hue fades in over the course of a few seconds and stays there until I close the window. If I immediately return to the previously blushing site, the color is gone. (If I simply open another window and briefly navigate away from the blushing site and then go back, the color is still there.)

It doesn't happen at every site and it seems to make no difference whether I've got one window open or many. What the nut? I've found no mention of this phenomenon anywhere.