Server transforms

Some apps run one more pass over the HTML after rendering, such as a server render for web components, a translation proxy or a tool that edits markup. Islands make this difficult, because an island hydrates against the exact markup Svelte rendered. A pass that moves a node, trims a space or wraps a child breaks the hydration walk, and the island throws its server DOM away.

transform lets that pass run anyway. You hand it to ogygia, which marks the HTML before your transform runs, reads what the transform did afterwards, and in the browser restores exactly what hydration needs before anything hydrates.

// src/hooks.server.ts
import * as ogygia from 'ogygia/server';
import { renderComponents } from '$lib/server/components';

export const handle = ogygia.handle({
  transform: async (html, { kind, csr, event }) => renderComponents(html, { fragment: kind === 'region' })
});

The transform runs on:

• every document, meaning the chunk that carries </head>. A streamed page’s tail (the scripts that settle its streamed promises) passes around it untouched, so the page keeps streaming.
• every region answer, such as a server island’s answer or a lake remount. In that case kind is 'region'.

It may be async. Because it sits on the response path, keep it fast.

What a transform may do

You can rewrite anything outside the custom elements that ogygia marks with og-h. ogygia puts og-h on each custom element (a tag with a - in its name) that it will restore:

• one in markup Svelte owns (an island, or the whole body of a Kit-hydrated page);
• one in server-owned markup with Svelte-owned markup below it (a component around an island in a lake);
• every one in a region answer, since the page morphs an answer over live elements.

A host without og-h is entirely yours: render it however you like, and let its own runtime adopt it later.

Rendering an og-h host: its plan

To reshape an og-h host, render it in “scoped” form with three additions:

<my-button og-h="7" og-shadow="my-button.shadow" og-keep="icon-name" icon-name="star" class="sc-my-button-h">
  <span class="btn sc-my-button">
    <slot class="sc-my-button sc-my-button-s"><!--og-c 7.0-->Sign in</slot>
  </span>
</my-button>

• <slot> wrappers. Where the component would slot the host’s children, wrap them in a real <slot> element. Outside a shadow root a <slot> renders as if it were not there, so the page paints exactly like your scoped render. Give it the classes any other element of the tree would have, and put slot fallback content inside it, before the children.
• og-shadow="key …" names the keyed stylesheets for the host’s shadow root (see Stylesheets).
• og-keep="attr …" (optional) lists attributes you added to the host that must survive the restore, for example an attribute your component reads before the island hydrates. A kept attribute must stay true of the live element at all times rather than describing one render, because a hole answer copies attributes onto live elements.

An og-h host without og-shadow is left exactly as Svelte wrote it, which is what you want for components you don’t render on the server.

Every other og-* mark belongs to ogygia (og-c on each child, og-t before text a serializer might trim). Carry these marks along wherever you move a node; none of them reaches the final DOM.

A mark comment sits immediately before its node. If your renderer marks each node it moves with a comment of its own (a relocation marker), don’t put that marker between ogygia’s mark and its node, at least on og-h hosts. Otherwise ogygia reads the node as dropped, and the restore puts it back twice.

A transform that caches its renders

ogygia numbers hosts across the whole document, so the same block carries different ids on another page, and a render cache keyed on the block’s markup would never hit. localizeMarks renumbers a block’s marks from 0 for the cache key, and maps the document’s ids back onto the render:

import { localizeMarks } from 'ogygia/markup';

const local = localizeMarks(block);
const rendered = cache.get(local.html) ?? cache.set(local.html, await render(local.html)).get(local.html);
return local.back(rendered);

Stylesheets

Tag head assets with data-og-head="key", anywhere in a document or an answer:

• <style data-og-head="…"> or <link data-og-head="…"> applies to the page;
• <template data-og-head="…">css</template> is a shadow sheet, built once into a shared CSSStyleSheet and adopted by every root that names its key in og-shadow.

ogygia moves them into <head>, once per key, and keeps all their other attributes.

What ogygia restores, and when

For each planned host, innermost first and in one task, ogygia restores:

1. an open shadow root, with its keyed sheets;
2. the rendered tree, moved into that root, so its <slot> wrappers become real slots;
3. Svelte’s children back in the light DOM, in order and exactly as Svelte rendered them, with attributes your render added removed and text it trimmed or dropped put back;
4. the host’s attributes, back to Svelte’s except for those listed in og-keep.

The restore runs before anything else reads the markup:

• for the first document, as an inline script at the end of the body, so before any island wakes, before Kit starts and before a deferred module script upgrades a custom element;
• for a region answer, on the answer before it goes into the page (there is no paint in between, and nothing upgrades in an inert fragment);
• for a router swap, on the incoming page before the swap.

The restore always creates the shadow root itself. A host that already has one by the time the restore reaches it was upgraded first, so it is left as served, with an ogygia:restore-late event, and is never restored into a root that the component’s own constructor made.

Tags the HTML parser would restructure

Your tree lives in the light DOM, so the HTML parser applies its rules to it in the page’s context: an <a> inside the page’s <a> closes the outer link, and a <div> inside a <p> closes the paragraph. After your transform, ogygia finds every start tag the parser would restructure, where your render produced either the tag or the element it would close, and ships it as a stand-in (<og-as tag="a">) with a zero-specificity stylesheet that preserves its box. The restore turns each stand-in back into the real element. Misnesting that comes from the page itself is left alone.

The dev check

In dev, ogygia sends each planned host’s original children along with the page, and after restoring every host the restore compares it with them: node types, tags, text, comments and attributes. class is compared as a set of tokens and style as its declarations, with either one treated as absent when empty, since a serializer’s rewrite of those paints the same. A difference is reported as an error in the console, naming the first node that differs, and as an ogygia:restore-mismatch event:

[ogygia] the transform broke hydration markup: <my-button> > [1]: Svelte has <i>, the restored markup has nothing

Testing a transform

ogygia/markup runs the same round trip outside the app, so a test can harvest markup and run your transform over it:

import { transformMarkup, checkScript, restore } from 'ogygia/markup';

const settled = await transformMarkup(islandHtml, renderComponents, { kind: 'region', dev: true });
const tpl = document.createElement('template');
// (the dev check's payload in front: restore then compares each host with Svelte's markup)
tpl.innerHTML = checkScript(settled.check ?? {}) + settled.html;
restore(tpl.content);
// tpl.content now holds what hydration will see

restore runs in jsdom as is; without adoptable stylesheets, it gives each shadow root a <style> element instead.

ogygia/markup also provides scanRegions, liftRegions and restoreRegions for a pass that needs to know where the regions are. It imports nothing, so it runs anywhere.