This can easily be replaced by the following:
let template = document.createElement('template')
template.innerHTML = `<style>
:host{
…
}
</style>
<slot></slot>
`
// in your web component
shadowRoot.appendChild(document.importNode(template.content, true))
Create a template node from a template string
$ npm install make-template
let template = makeTemplate`<style>
div{
color: blue;
}
</style>
<div>Some text</div>`
console.log(template) // <template>…</template>