a DIV (HTMLDivElement) is a block element.

But you don't even need a defined Custom Element/Web Component to make a block element

customElements.define("currency-list", class extends HTMLElement {
  connectedCallback() {
    this.style.display = "block";
  }
});

another-list {
  display: block;
}

body>*:defined {
  background: green;
  color: beige;
}

body>*:not(:defined) {
  background: lightgreen;
}

Line 1
<currency-list>Hello Web Component</currency-list>
Line 3
<div>Line 4</div>
Line 5
<another-list onclick="alert(this.constructor.name)">Line 6</another-list>
Line 7

Notes:

• <currency-list> is an "Autonomous Custom Element" (extends HTMLElement)

• You can extend HTMLDivElement in Chromium and FireFox, but Apple has stated they will never implement "Customized Built-In Elements"
  From: https://github.com/WICG/webcomponents/issues/509

• <another-list> is an HTMLUnknownElement; nothing wrong with using it, its constructor is an HTMLElement, so can do everything an HTMLElement can do.
  For more on the value of Unknown Elements see my Dev.to post

• You can set any CSS display value on a DIV, you can't on your own Elements, as it will destroy the display:block setting.

PS. tag your SO questions web-component and/or custom-element