feat: support TrustedHTML in {@html} expressions

[Rich-Harris]

Follow-up to #16271.

Summary

• Allow {@html} blocks to accept TrustedHTML objects (from TrustedTypes policies) without coercing them to strings
• This enables usage like {@html myPolicy.createHTML(someHTML)}
• Works in regular HTML, SVG, and MathML contexts

Changes

• html.js: Instead of calling create_fragment_from_html, create the wrapper element directly (<template>, <svg>, or <math> depending on context) and assign the value to innerHTML. This preserves TrustedHTML objects.
• reconciler.js: Removed the trusted parameter from create_fragment_from_html since it's no longer used by {@html} and all remaining callers want trusted HTML.
• template.js and snippet.js: Removed the second argument from create_fragment_from_html calls.

Notes

No tests added because JSDOM doesn't implement TrustedTypes.

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🦋 Changeset detected

Latest commit: 981156b

The changes in this PR will be included in the next version bump.

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svelte	Minor

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Playground

pnpm add https://pkg.pr.new/svelte@17701

[7nik]

Btw, I believe these lines need trustifiing as well

svelte/packages/svelte/src/internal/client/dom/template.js

Lines 288 to 292 in 220b526

	for (var attribute of script.attributes) {
	clone.setAttribute(attribute.name, attribute.value);
	}
	clone.textContent = script.textContent;

svelte/packages/svelte/src/internal/client/dom/elements/customizable-select.js

Line 17 in 220b526

select.innerHTML = '<option><span>t</span></option>';

Edit: also, guess fragment_from_tree will fail assembling both inline-script and script with src.

[Rich-Harris]

Ah yep good catch. Those should probably go in a separate PR since it's separate to the {@html} stuff

[dummdidumm]

code looks good. Is this referencing any issue or PR? Description references none

[Rich-Harris]

ah yep, is a follow-up to #16271. Have updated the PR description

[Rich-Harris]

opened #17743 for #17701 (comment). Assigning textContent doesn't require trusted types (at least, AFAICT). It doesn't address the fragment_from_tree stuff, that can happen separately since it affects a different set of users

[7nik]

Assigning textContent doesn't require trusted types (at least, AFAICT).

Seems it requires in case of <script>.
HTMLScriptElement.textContent and HTMLScriptElement.src (plus .setAttribute("src", ...)) are listed as injection sinks.
Also: https://developer.mozilla.org/en-US/docs/Web/API/Trusted_Types_API#indirect_injection_sinks