In their paper [The spectral drop problem][1], Buttazzo and Velichkov state that the embedding $H_0^1(\Omega, D) \hookrightarrow L^2(D)$ is compact, where $\Omega \subset D$ is quasi-open, $D \subset \mathbb R^N$ is a Lipschitz open domain and $|\Omega| < |D|$ and 
$$
H_0^1(\Omega, D) = \{u \in H^1(D) \ : \ u = 0 \text{ quasi-everywhere in } D \setminus \Omega\}.
$$

They claim that this is a well-known fact and follows from the Lipschitz condition on $D$, but I wasn't able to find a proof.

At first I thought about adapting the classical proof of the Rellich-Kondrachov theorem, but I failed.

Can you recommend any reference where I can find a detailed proof of this fact, or any hints?

I am beginning to learn about capacities and quasi-things, and would like to see the proof to get more acquainted with the theory.

Thanks in advance.

  [1]: https://arxiv.org/abs/1406.1627