Inside a declarative shadow root, a <style> uses :host selectors and the host has reflected attributes set in the same parsed markup. At the moment the parser materializes the shadow root, which is true about the interaction between declarative shadow DOM and the upgrade of a still-undefined custom element host?
<x-widget loud>
<template shadowrootmode="open">
<style>:host([loud]) { font-weight: 700 }</style>
<slot></slot>
</template>
</x-widget>
<!-- x-widget defined later via customElements.define -->