Styling a {@html...} tag of a Svelte component by ising the in-component <style> tag (Unused CSS selector)

You need to use the :global(...) modifier. If you do this...

:global(h1) { color: blue }

...it will apply to all <h1> elements on the page — both the one inside the {@html ...} tag and any that exist in other components.

To restrict it to elements inside this component, put the selector inside a local one:

p :global(h1) { color: blue }

That selector will only apply to <h1> elements inside <p> elements belonging to this component. Here's a demo