What model? If an iMac it could be either LCD or video card if it does it again in other apps too.
Was it all over the screen or just in the Safari window?
If not an iMac, video card. Looks more like video card, bad LCDs usually have solid stripes.
Or like Michael said, just corrupted data for that page. Have to wait and see if it happens again, and in other apps, or on the desktop.