Get list of heading nodes for a page during SSR

[janosh]

Describe the problem

Say I want to create a custom table of contents that doesn't flash on page load, is there any way to get a list of headings on a page during SSR and return it from the module script's load function?

Essentially this but of course without using unavailable browser globals:

<script lang="ts" context="module">
  import type { Load } from '@sveltejs/kit'

  export const load: Load = () => {
    const headings = document.querySelectorAll(`h1, h2, h3`)
    return { props: headings }
  }
</script>

<script lang="ts">
  export let headings: HTMLHeadingElement[]

  console.log(headings)
</script>

Describe the proposed solution

No idea.

Alternatives considered

No response

Importance

nice to have

Additional Information

No response

[janosh]

Related discussion: pngwn/MDsveX#268