Step 1: Template, Props, and Attributes ​

import { LitElement, html } from 'lit-element'

export class CustomAlert extends LitElement {
  static get properties() { 
    return {
      icon: { type: String },
      theme: { type: String },
    }
  }
  constructor() {
    super()
    this.setAttribute('role', 'alert’)
    this.setAttribute('aria-live', 'polite’)
  }
  render() {
    return html`
      <svg xmlns="http://www.w3.org/2000/svg" style="display: none;">
        <symbol id="note" viewBox="0 0 24 24" fill="none" stroke="currentColor">
          <path
            strokeLinecap="round"
            strokeLinejoin="round"
            strokeWidth="2"
            d="M7 8h10M7 12h4m1 8l-4-4H5a2 2 0 01-2-2V6a2 2 0 012-2h14a2 2 0 012 2v8a2 2 0 01-2 2h-3l-4 4z"
          />
        </symbol>
        <symbol id="warn" viewBox="0 0 24 24" fill="none" stroke="currentColor">
          <path
            stroke-linecap="round"
            stroke-linejoin="round"
            stroke-width="2"
            d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-3L13.732 4c-.77-1.333-2.694-1.333-3.464 0L3.34 16c-.77 1.333.192 3 1.732 3z"
          />
        </symbol>
      </svg>
      
      <svg part="icon" height="24" width="24"><use href="#${this.icon}"></use></svg>
      <slot></slot>
      <button part="button">Close</button>
    `
  }
}
window.customElements.define('custom-alert', CustomAlert)

import { LitElement, html } from 'lit-element'

export class CustomAlert extends LitElement {
  static get properties() { 
    return {
      icon: { type: String },
      theme: { type: String },
    }
  }
  constructor() {
    super()
    this.setAttribute('role', 'alert’)
    this.setAttribute('aria-live', 'polite’)
  }
  render() {
    return html`
      <svg xmlns="http://www.w3.org/2000/svg" style="display: none;">
        <symbol id="note" viewBox="0 0 24 24" fill="none" stroke="currentColor">
          <path
            strokeLinecap="round"
            strokeLinejoin="round"
            strokeWidth="2"
            d="M7 8h10M7 12h4m1 8l-4-4H5a2 2 0 01-2-2V6a2 2 0 012-2h14a2 2 0 012 2v8a2 2 0 01-2 2h-3l-4 4z"
          />
        </symbol>
        <symbol id="warn" viewBox="0 0 24 24" fill="none" stroke="currentColor">
          <path
            stroke-linecap="round"
            stroke-linejoin="round"
            stroke-width="2"
            d="M12 9v2m0 4h.01m-6.938 4h13.856c1.54 0 2.502-1.667 1.732-3L13.732 4c-.77-1.333-2.694-1.333-3.464 0L3.34 16c-.77 1.333.192 3 1.732 3z"
          />
        </symbol>
      </svg>
      
      <svg part="icon" height="24" width="24"><use href="#${this.icon}"></use></svg>
      <slot></slot>
      <button part="button">Close</button>
    `
  }
}
window.customElements.define('custom-alert', CustomAlert)