Spiderlings

I noticed this hanging on a windowsill today. It looked like a bundle of tiny yellow spheres in a web bag; then I noticed that they were moving. They turned out to be spiderlings. My photo doesn't do them justice, but they were tiny and wouldn't keep cooperate and keep still. I don't know what species they are - Google didn't help, alas. Anyone know their spiders; bright yellow back, transparent legs and a triangular black mark on the rear?