The Owl House/WMG

The show takes place in the same universe as Little Witch Academia
The two series has way too many in common to not share the same universe. Both are witch works. Both have Wizarding Schools. Both have Inept Mages starting in said wizarding school. In fact, many characters from Little Witch Academia are directly Expyed into The Owl House. Luz took traits from Atsuko Kagari, Amity from the Defrosting Ice Queen Diana Cavendish, Azura from Shiny Chariot, and Willow is a combination of Sucy and Lotte. Little Witch Academia takes place in the human world in The Owl House. The Boiling Isles is an alternative reality of the events occurring within Little Witch Academia. Every event that happened to the wizards in the real world has a version that's mirrored in the Boiling Isles. Why is Luz such a fan of witches and fantasy? Because she witnessed Shiny Chariot's magic show in person, and wants to pursuit magic for herself. Halfway through episode 1 of season 3, there is a Luna Nova uniform hanging on a wall--further evidence that Luna Nova exists in the world of The Owl House.

In Howl's Moving Castle, the black dial on Howl's door leads to the Boiling Isles
In Howl's Moving Castle, the titular moving castle has a door with four destinations. The destinations lead across space and time. One of these destinations is the Boiling Isles.

Eda's curse is similar to Howl's curse. Both transform them into giant bird creatures, amplifying their power but turning them into primitive beings. It's safe to say Howl's curse spread to Eda. A scholar studied Howl, and wrote instructions for his curse down on a scroll. That's how the curse entered the Boiling Isles. Maybe Eda's shack is where Howl's door led before she took over!

In the original book, Wales is hidden behind a think layer of pure blackness in the black dial. The same thing happens in the film. Although we are shown the darkness and war immediately beyond the dial, if you venture further you end up in the Boiling Isles. Both works made it clear they feature many alternative dimensions. One of the dimensions is the other work's world.