docs(faq): explain the runtime-DOM-mutation false-invalid case#4
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Adds an FAQ entry for the failure mode we hit on this site: the Hugo Blox theme injects copy buttons into <pre> blocks at runtime, which makes the live-DOM innerHTML disagree with the static HTML the signer hashed. The signature is mathematically correct, but the verifier reads a different canonical text and reports invalid. Documents the three practical workarounds (configure/patch the theme, pre-render decoration server-side, move decoration outside the signed region) and links to the matching Known Issue section in the spec README. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
Adds an FAQ entry explaining why a signed page can show as invalid even when the signature is mathematically correct: client-side JavaScript mutating content inside a
<signed-section>between page load and verification.We hit this on this site itself — the Hugo Blox docs theme injects
<button class=\"copy-button\">Copy</button>into every<pre>block at runtime. The signer never saw the button; the extension's verifier reads it from the live DOM and the canonical text diverges.Links to the matching Known Issue section in the spec README (separate PR: HTMLTrust/htmltrust-spec#2).
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