Comment by typedef_struct

You could start with something like this:

    customElements.define('html-import', class extends HTMLElement {
        connectedCallback() {
            const href = this.getAttribute('href')
            const fetch = new XMLHttpRequest()
            fetch.responseType = 'document'
            fetch.addEventListener('readystatechange', (function onfetch(e) {
                if (fetch.readyState !== XMLHttpRequest.DONE) return
                const document = fetch.response.querySelector('body') ?? fetch.response
                this.replaceWith(document)
            }).bind(this))
            fetch.open('GET', href)
            fetch.send()
        }
    })