MyElement.js

import { render, html } from 'lit-html'

class MyElement extends HTMLElement {
    constructor(){
        super()

        this.state = {
            loading: true
        }
    }

    setState(stateUpdate) {
        const newState = {
            ...this.state,
            ...stateUpdate
        }
        if (!isEqual(newState, this.state)) {
            this.state = newState
            this.render()
        }
    }

    connectedCallback(){
        this.render()

        setTimeout(() => {
            this.setState({
                loading: false
            })
        }, 2000)
    }

    render(){
        const { loading } = this.state

        const content = html`
        <h1>${loading ? ('Loading') : ('Done')}</h1>
        `

        render(content, this)
    }

}

customElements.define('my-element', MyElement)