Magma ring mob XP farm

A beginner mob XP farm by MCBlanky, ten spawning platforms stacked in a column with dispensers flushing each one on a note block clock, and a ring of magma blocks at the bottom finishing off whatever survives the fall. A slab ceiling keeps it running in daylight. It is cheap, mostly smooth stone and slabs, and the design is credited loosely to Jumper, Voltrox, Moretingz, RealJayden and BlazedMC. The build is well liked; the tutorial is not, mainly because it ships no item list and claims Bedrock support it does not have.

• It does not work on Bedrock despite the tags. gimpygonzales lists why: observers do not register note block updates, there is no quasi connectivity so an observer cannot fire a dispenser a block away, and the ceiling has to be full blocks because light passes through slabs.
• salinasmaster4955 has the shortest Bedrock fix that works: swap the note block for a dispenser and put a mangrove root between each dispenser and observer.
• Add a third ring of magma blocks. osha8959 and Yodude9374 both found mobs landing on the fences and surviving, and one surviving creeper wrecks the collection.
• If nothing spawns, stand closer to the magma blocks. xpz6750 calls that the sweet spot, and TootyTootyTootyTootyTooty adds eight more repeaters to the clock when the timing is off.
• MCBlanky's own pinned correction: it will not work in a biome that cannot spawn hostile mobs, mushroom islands included.
• The slab ceiling is what lets it run in daylight rather than only at night, which lonewolf786-12 explains to everyone who assumes otherwise.