You can use <web-greeting>
programatically:
const webGreeting = document.createElement("web-greeting")
webGreeting.name = "Justin"
document.body.append(webGreeting)
webGreeting.innerHTML //-> "<h1>Hello, Justin</h1>"
Or you can use it declaratively:
document.body.innerHTML = "<web-greeting></web-greeting>"
document.body.firstChild.name = "I do declare"
document.body.firstChild.innerHTML //-> "<h1>Hello, I do declare</h1>"