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docs(faq): explain the runtime-DOM-mutation false-invalid case #4

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jt55401 merged 1 commit into from
May 14, 2026
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docs(faq): explain the runtime-DOM-mutation false-invalid case #4

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20 changes: 20 additions & 0 deletions content/faq/_index.md
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Expand Up @@ -58,6 +58,26 @@ Yes — that is a core feature. Each `<signed-section>` is independent, so a sin

HTMLTrust degrades gracefully. Unsigned or unrecognized `<signed-section>` elements render as normal content. Only the verification features are unavailable. The page remains fully readable and functional.

## Why does a signed page show as *invalid* in the browser even when the signature is mathematically correct?

This is a real, observed issue: HTMLTrust signs the **static HTML** that leaves the publishing pipeline, but the browser extension verifies against the **live DOM** — the state of the page after every script on the page has finished running. If anything inside a `<signed-section>` is mutated between page load and verification, the recomputed `content-hash` won't match the signed one, and the extension reports an invalid signature.

We hit this on our own site initially: the Hugo Blox docs theme injects a `<button class="copy-button">Copy</button>` into every `<pre>` code block at runtime. The signer never saw the button, so the canonical text the verifier reads includes extra "Copy" tokens that aren't in the hash.

Other common culprits:

- Client-side syntax highlighters (Prism, highlight.js) that rewrite code blocks
- Lazy-loading or share-button widgets that add nodes inside article content
- Theme JS that decorates headings, callouts, or admonitions at runtime

**What to do about it** — until the spec defines a mutation-skip marker, the practical workarounds are:

1. Configure or patch the theme so it does **not** inject nodes inside `<signed-section>` descendants
2. Pre-render any decoration server-side, so the signer hashes it
3. Move runtime-injected decoration **outside** the signed region (as a sibling, not a child)

This is being tracked as a spec open design question — see [Known Issue: Runtime DOM Mutation](https://github.com/HTMLTrust/htmltrust-spec#known-issue-runtime-dom-mutation-breaks-verification) in the spec README.

## What browsers are supported?

A reference browser extension is available for **Chrome** (and Chromium-based browsers like Edge). Firefox and Safari support is planned. Eventually, the goal is native browser support via a W3C standard.
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