Here’s an HTML-only check for Declarative Shadow DOM support that I used in a demo just now:

```
<div>
<p>Declarative Shadow DOM is <strong>⚠️ not supported</strong> in your browser.</p>
<template shadowrootmode="open">
<p>Declarative Shadow DOM <strong>✅ is supported</strong> in your browser.</p>
</template>
</div>
```