When standing on top of a half slab in broad daylight, skeletons (i assume zombies too) will not catch on fire. Once they step off the half slab, they will catch on fire. I spawned two skeletons next to each other at day, one on a slab one on the ground, and the one on the slab stayed completely safe as the other burned.I haven't tested it in the regular client yet, so it may be a client issue, but I'm not sure.