DOM Based Class System

class MyElement extends UIElement {
    template () {
        return `<div>my element</div>`
    }
}

Use the template () method to specify the actual HTML string for MyElement.

A UIElement can be contained in other UIElement.

class SecondElement extends UIElement {
    components () {
        return { MyElement }
    }
    template () {
        return `
        <div>
            <object refClass='MyElement' />
        </div>
        `
    }
}

It creates MyElement internally when SecondElement is created. At this time, the parent property of MyElement becomes the instance of SecondElement.