While at Ron and Steve's, showed some of my web-based photos. Some came out rotated by 90°, something I've never seen before, such as this one:
The issue appears to be this EXIF tag:
Orientation : Rotate 270 CW
That's a left-over from the way I took the images: camera mounted vertically, stitched together to make a landscape image, such as this one:
I didn't know of any web browser that evaluated the EXIF data. Now it looks as if I'll have to reprocess thousands of images to remove that tag.