diff --git a/web/src/lib/components/editor/Editor.svelte b/web/src/lib/components/editor/Editor.svelte
index 5b1ecd81..7b566b8e 100644
--- a/web/src/lib/components/editor/Editor.svelte
+++ b/web/src/lib/components/editor/Editor.svelte
@@ -1385,7 +1385,79 @@
 		border-color: var(--accent-blue);
 	}
 
-
+	/* Hidden-content authoring warning (TASK-1327 / PLAN-1322) */
+	.editor-content :global(.html-block-warning) {
+		display: block;
+		width: 100%;
+		text-align: left;
+		padding: var(--space-2) var(--space-3);
+		background: var(--bg-warning, rgba(255, 180, 50, 0.12));
+		border: none;
+		border-bottom: 1px solid var(--border);
+		color: var(--accent-orange, #d68a3a);
+		font-size: 0.85em;
+		font-family: var(--font-mono);
+		cursor: pointer;
+		transition: background 0.12s;
+	}
+	.editor-content :global(.html-block-warning:hover) {
+		background: var(--bg-warning-hover, rgba(255, 180, 50, 0.22));
+	}
+	.editor-content :global(.html-block-inspector) {
+		padding: var(--space-3);
+		border-top: 1px solid var(--border);
+		background: var(--bg-secondary);
+		font-size: 0.9em;
+	}
+	.editor-content :global(.html-block-inspector-heading) {
+		font-weight: 600;
+		color: var(--text-primary);
+		margin-bottom: var(--space-2);
+	}
+	.editor-content :global(.html-block-inspector-list) {
+		list-style: none;
+		padding: 0;
+		margin: 0 0 var(--space-2) 0;
+	}
+	.editor-content :global(.html-block-inspector-item) {
+		padding: var(--space-2) 0;
+		border-bottom: 1px solid var(--border);
+	}
+	.editor-content :global(.html-block-inspector-item:last-child) {
+		border-bottom: none;
+	}
+	.editor-content :global(.html-block-inspector-rule) {
+		color: var(--text-secondary);
+	}
+	.editor-content :global(.html-block-inspector-rule code) {
+		color: var(--accent-blue);
+		font-family: var(--font-mono);
+		font-size: 0.95em;
+	}
+	.editor-content :global(.html-block-inspector-snippet) {
+		margin-top: 4px;
+		padding: 4px var(--space-2);
+		color: var(--text-muted);
+		font-family: var(--font-mono);
+		font-size: 0.85em;
+		background: var(--bg-tertiary);
+		border-radius: var(--radius-sm);
+		overflow-wrap: anywhere;
+	}
+	.editor-content :global(.html-block-dismiss) {
+		padding: 4px 12px;
+		font-size: 0.85em;
+		background: var(--bg-tertiary);
+		border: 1px solid var(--border);
+		border-radius: var(--radius-sm);
+		color: var(--text-secondary);
+		cursor: pointer;
+		transition: color 0.12s, border-color 0.12s;
+	}
+	.editor-content :global(.html-block-dismiss:hover) {
+		color: var(--text-primary);
+		border-color: var(--accent-blue);
+	}
 
 	/* Mobile keyboard toolbar */
 	.mobile-toolbar {
diff --git a/web/src/lib/components/editor/extensions/htmlBlock.ts b/web/src/lib/components/editor/extensions/htmlBlock.ts
index d8c5c64b..c7e86c3e 100644
--- a/web/src/lib/components/editor/extensions/htmlBlock.ts
+++ b/web/src/lib/components/editor/extensions/htmlBlock.ts
@@ -19,6 +19,12 @@ import { InputRule, Node, type Editor } from '@tiptap/core';
 import type MarkdownIt from 'markdown-it';
 import type { Node as ProseMirrorNode } from '@tiptap/pm/model';
 import { sanitizeHtmlBlock } from '$lib/utils/markdown';
+import {
+	detectHiddenContent,
+	isHiddenContentAcknowledged,
+	setHiddenContentAcknowledged,
+	type HiddenSegment,
+} from '$lib/utils/hiddenContentDetector';
 
 /** A single htmlBlock node's identity for snapshot comparison. */
 export interface HtmlBlockSnapshotEntry {
@@ -219,10 +225,91 @@ export const HtmlBlock = Node.create({
 
 			actions.append(doneBtn);
 			source.append(textarea, actions);
-			wrapper.append(preview, source);
+
+			// Hidden-content authoring warning (TASK-1327). The pill is
+			// always created but only revealed when detectHiddenContent
+			// finds segments AND the user hasn't dismissed for this block.
+			const warning = document.createElement('button');
+			warning.type = 'button';
+			warning.className = 'html-block-warning';
+			warning.hidden = true;
+			warning.title = 'Click to inspect hidden segments in this block';
+
+			const inspector = document.createElement('div');
+			inspector.className = 'html-block-inspector';
+			inspector.hidden = true;
+
+			wrapper.append(warning, preview, source, inspector);
 
 			let lastHtml = (node.attrs.html as string | undefined) ?? '';
 			let mode: 'preview' | 'source' = 'preview';
+			let lastSegments: HiddenSegment[] = [];
+
+			const renderInspector = () => {
+				if (lastSegments.length === 0) {
+					inspector.replaceChildren();
+					return;
+				}
+				const heading = document.createElement('div');
+				heading.className = 'html-block-inspector-heading';
+				heading.textContent = `${lastSegments.length} hidden segment${lastSegments.length === 1 ? '' : 's'}`;
+
+				const list = document.createElement('ul');
+				list.className = 'html-block-inspector-list';
+				for (const seg of lastSegments) {
+					const item = document.createElement('li');
+					item.className = 'html-block-inspector-item';
+					const label = document.createElement('div');
+					label.className = 'html-block-inspector-rule';
+					const tagSpan = document.createElement('code');
+					tagSpan.textContent = seg.tag;
+					label.append(tagSpan, document.createTextNode(` — ${seg.rule}`));
+					item.appendChild(label);
+					if (seg.snippet) {
+						const snip = document.createElement('div');
+						snip.className = 'html-block-inspector-snippet';
+						snip.textContent = `"${seg.snippet}"`;
+						item.appendChild(snip);
+					}
+					list.appendChild(item);
+				}
+
+				const dismiss = document.createElement('button');
+				dismiss.type = 'button';
+				dismiss.className = 'html-block-dismiss';
+				dismiss.textContent = 'Dismiss for this block';
+				dismiss.title = 'Mark these segments as reviewed; warning will not re-appear';
+				// mousedown.preventDefault keeps focus where it was so the
+				// click handler runs in-context.
+				dismiss.addEventListener('mousedown', (e) => e.preventDefault());
+				dismiss.addEventListener('click', (e) => {
+					e.preventDefault();
+					if (!editor.isEditable) return;
+					const pos = typeof getPos === 'function' ? getPos() : null;
+					if (typeof pos !== 'number') return;
+					const next = setHiddenContentAcknowledged(lastHtml);
+					if (next === lastHtml) return;
+					editor.view.dispatch(
+						editor.view.state.tr.setNodeMarkup(pos, undefined, { html: next }),
+					);
+				});
+
+				inspector.replaceChildren(heading, list, dismiss);
+			};
+
+			const updateWarning = () => {
+				lastSegments = detectHiddenContent(lastHtml);
+				const acked = isHiddenContentAcknowledged(lastHtml);
+				if (lastSegments.length === 0 || acked) {
+					warning.hidden = true;
+					inspector.hidden = true;
+					wrapper.classList.remove('html-block--has-hidden');
+					return;
+				}
+				warning.textContent = `⚠ ${lastSegments.length} hidden segment${lastSegments.length === 1 ? '' : 's'} — click to inspect`;
+				warning.hidden = false;
+				wrapper.classList.add('html-block--has-hidden');
+			};
 
 			const renderPreview = () => {
 				if (!lastHtml.trim()) {
@@ -234,9 +321,17 @@ export const HtmlBlock = Node.create({
 				} else {
 					preview.innerHTML = sanitizeHtmlBlock(lastHtml);
 				}
+				updateWarning();
+				renderInspector();
 			};
 			renderPreview();
 
+			warning.addEventListener('click', (e) => {
+				e.preventDefault();
+				e.stopPropagation();
+				inspector.hidden = !inspector.hidden;
+			});
+
 			function flipToSource() {
 				if (mode === 'source') return;
 				mode = 'source';
diff --git a/web/src/lib/utils/hiddenContentDetector.ts b/web/src/lib/utils/hiddenContentDetector.ts
new file mode 100644
index 00000000..4733cfd6
--- /dev/null
+++ b/web/src/lib/utils/hiddenContentDetector.ts
@@ -0,0 +1,247 @@
+/**
+ * Hidden-content detector for HTML blocks (TASK-1327 / PLAN-1322).
+ *
+ * Walks an HTML string looking for elements + comments that are visible
+ * to LLMs / agents reading the document but invisible (or near-invisible)
+ * to a human author reviewing the rendered preview. The detector is an
+ * **authoring-honesty** feature — NOT a security control. Render-time
+ * sanitization (sanitizeHtmlBlock) protects browsers; this protects
+ * authors from accidentally pasting content that looks one way and
+ * reads another.
+ *
+ * Heuristics flag content that is one of:
+ *   - CSS-hidden via inline style (display:none, visibility:hidden,
+ *     opacity:0, font-size:0 or very small, color === background-color)
+ *   - Off-screen positioned (position:absolute with very negative
+ *     left/right/top/bottom, transform translate to off-screen)
+ *   - Zero-dimension (width:0 AND height:0, clip:rect(0,0,0,0))
+ *   - HTML comments
+ *   - Suspiciously long aria-label / alt / title (>200 chars or
+ *     containing newlines — heuristic for hidden text in attribute
+ *     values)
+ *
+ * Class-based hiding (e.g. `.sr-only`) is NOT flagged: resolving it
+ * requires the page's stylesheet context, which we don't have here,
+ * and the false-positive rate would be too high. The detector stays
+ * conservative.
+ */
+
+/**
+ * The Pad-internal sentinel used by `setHiddenContentAcknowledged` to
+ * mark that a human has reviewed and accepted the hidden content.
+ * `detectHiddenContent` skips this exact comment so dismissal sticks
+ * after a doc reload.
+ */
+export const PAD_ACK_HIDDEN_MARKER = '<!-- pad:ack-hidden -->';
+
+const PAD_ACK_HIDDEN_TEXT = ' pad:ack-hidden '; // textContent of the marker (with surrounding spaces)
+const PAD_ACK_HIDDEN_TEXT_TRIMMED = 'pad:ack-hidden';
+
+export interface HiddenSegment {
+	/** Lowercase tag name of the offending element, or `#comment`. */
+	tag: string;
+	/** Short human-readable rule label, e.g. `display:none` or `font-size:2px (too small)`. */
+	rule: string;
+	/** Up to 80 chars of trimmed text content, for context in the inspector. */
+	snippet: string;
+}
+
+/**
+ * Returns true when the user has previously dismissed the hidden-content
+ * warning for this block. The marker lives at the top of the html string
+ * so it's the first thing read on parse and survives markdown round-trips
+ * (it's just an HTML comment, valid in any context where raw HTML is
+ * permitted).
+ */
+export function isHiddenContentAcknowledged(html: string): boolean {
+	return html.trimStart().startsWith(PAD_ACK_HIDDEN_MARKER);
+}
+
+/**
+ * Prepend the ack marker if not already present. Idempotent. Used by the
+ * NodeView's "Dismiss for this block" affordance.
+ */
+export function setHiddenContentAcknowledged(html: string): string {
+	if (isHiddenContentAcknowledged(html)) return html;
+	const sep = html.startsWith('\n') || html === '' ? '' : '\n';
+	return `${PAD_ACK_HIDDEN_MARKER}${sep}${html}`;
+}
+
+/**
+ * Detect hidden content in an HTML block's raw source. Returns one
+ * `HiddenSegment` per offending element / attribute / comment. Returns
+ * an empty array when running outside a browser (no DOMParser) — the
+ * detector is purely client-side.
+ */
+export function detectHiddenContent(html: string): HiddenSegment[] {
+	if (typeof window === 'undefined' || typeof DOMParser === 'undefined') return [];
+	if (!html) return [];
+
+	const segments: HiddenSegment[] = [];
+	const doc = new DOMParser().parseFromString(html, 'text/html');
+
+	// Walk all elements regardless of whether the parser placed them
+	// inside body (the typical case) or moved them to head (e.g. <style>
+	// / <link> at the top of the snippet). We only check elements that
+	// rendered content can hide behind, but element-level checks are
+	// cheap so just walk the whole document.
+	doc.querySelectorAll('*').forEach((el) => {
+		// Skip <html>, <head>, <body> wrappers DOMParser inserts; they
+		// never carry the inline-style hide patterns we look for.
+		const tag = el.tagName.toLowerCase();
+		if (tag === 'html' || tag === 'head' || tag === 'body') return;
+		segments.push(...checkElement(el as HTMLElement));
+	});
+
+	// Walk comments anywhere in the document. DOMParser can place
+	// comments at document level (siblings of <html>) when the input
+	// has a leading `<!-- ... -->`; walking only doc.body would miss
+	// those. createTreeWalker on `doc` is the inclusive root.
+	const walker = doc.createTreeWalker(doc, NodeFilter.SHOW_COMMENT);
+	let node = walker.nextNode();
+	while (node) {
+		const text = node.textContent ?? '';
+		const trimmed = text.trim();
+		if (text !== PAD_ACK_HIDDEN_TEXT && trimmed !== PAD_ACK_HIDDEN_TEXT_TRIMMED) {
+			segments.push({
+				tag: '#comment',
+				rule: 'HTML comment',
+				snippet: snippetFor(text),
+			});
+		}
+		node = walker.nextNode();
+	}
+
+	return segments;
+}
+
+function checkElement(el: HTMLElement): HiddenSegment[] {
+	const segments: HiddenSegment[] = [];
+	const tag = el.tagName.toLowerCase();
+	// `el.style` is the browser-parsed CSSStyleDeclaration for the
+	// inline `style` attribute. The browser handles every CSS edge
+	// case for us:
+	//
+	//   - `!important` priority (display:none!important;display:block
+	//     leaves `el.style.display === 'none'`)
+	//   - CSS comments (display:/**/none parses correctly)
+	//   - case-insensitive keywords (DISPLAY: NONE → 'none')
+	//   - whitespace and unit normalisation
+	//
+	// A hand-rolled `style` attribute parser misses these consistently;
+	// using the CSSOM here is materially more robust.
+	const style = el.style;
+	const text = el.textContent ?? '';
+
+	if (style.display === 'none') {
+		segments.push({ tag, rule: 'display:none', snippet: snippetFor(text) });
+	}
+	if (style.visibility === 'hidden') {
+		segments.push({ tag, rule: 'visibility:hidden', snippet: snippetFor(text) });
+	}
+	if (isZeroOpacity(style.opacity)) {
+		segments.push({ tag, rule: `opacity:${style.opacity}`, snippet: snippetFor(text) });
+	}
+	const fontSize = parseLength(style.fontSize);
+	if (fontSize !== null && fontSize < 6) {
+		segments.push({
+			tag,
+			rule: `font-size:${style.fontSize} (too small)`,
+			snippet: snippetFor(text),
+		});
+	}
+	if (
+		style.color &&
+		style.backgroundColor &&
+		normalizeColor(style.color) === normalizeColor(style.backgroundColor)
+	) {
+		segments.push({
+			tag,
+			rule: 'color matches background-color',
+			snippet: snippetFor(text),
+		});
+	}
+	if (style.position === 'absolute' || style.position === 'fixed') {
+		for (const prop of ['left', 'right', 'top', 'bottom'] as const) {
+			const value = style.getPropertyValue(prop);
+			const offset = parseLength(value);
+			if (offset !== null && offset <= -9000) {
+				segments.push({
+					tag,
+					rule: `${prop}:${value} (off-screen)`,
+					snippet: snippetFor(text),
+				});
+				break;
+			}
+		}
+	}
+	if (style.transform && /translate[xy]?\s*\(\s*-?\d+/i.test(style.transform)) {
+		const off = /-\s*9\d{3,}|-\s*\d{5,}/.test(style.transform);
+		if (off) {
+			segments.push({
+				tag,
+				rule: 'transform off-screen',
+				snippet: snippetFor(text),
+			});
+		}
+	}
+	const w = parseLength(style.width);
+	const h = parseLength(style.height);
+	if (w === 0 && h === 0) {
+		segments.push({ tag, rule: 'width:0;height:0', snippet: snippetFor(text) });
+	}
+	if (style.clip && /rect\(\s*0(?:px)?\s*,\s*0(?:px)?\s*,\s*0(?:px)?\s*,\s*0(?:px)?\s*\)/i.test(style.clip)) {
+		segments.push({ tag, rule: 'clip:rect(0,0,0,0)', snippet: snippetFor(text) });
+	}
+
+	for (const attr of ['aria-label', 'alt', 'title'] as const) {
+		const val = el.getAttribute(attr);
+		if (!val) continue;
+		const tooLong = val.length > 200;
+		const hasNewline = /\n/.test(val);
+		if (tooLong || hasNewline) {
+			const reasons: string[] = [];
+			if (tooLong) reasons.push(`${val.length} chars`);
+			if (hasNewline) reasons.push('contains newlines');
+			segments.push({
+				tag,
+				rule: `${attr} suspicious (${reasons.join(', ')})`,
+				snippet: snippetFor(val),
+			});
+		}
+	}
+
+	return segments;
+}
+
+/**
+ * Parse a CSS length declaration like `12px`, `0`, `-9999px`, `1.5em`
+ * into a number when the unit is bare (`0`) or px. Returns `null` for
+ * percentage / em / rem / unrecognised forms — those don't have a
+ * single numeric "is this hidden" answer.
+ */
+function parseLength(value: string | undefined): number | null {
+	if (!value) return null;
+	const trimmed = value.trim();
+	if (trimmed === '0') return 0;
+	const match = trimmed.match(/^(-?[\d.]+)px$/i);
+	if (!match) return null;
+	const n = parseFloat(match[1]);
+	return Number.isFinite(n) ? n : null;
+}
+
+function isZeroOpacity(value: string | undefined): boolean {
+	if (!value) return false;
+	const v = value.trim();
+	return v === '0' || v === '0.0' || v === '0%' || v === '.0';
+}
+
+function normalizeColor(c: string): string {
+	return c.toLowerCase().replace(/\s+/g, '');
+}
+
+function snippetFor(text: string): string {
+	const collapsed = text.trim().replace(/\s+/g, ' ');
+	if (collapsed.length <= 80) return collapsed;
+	return `${collapsed.slice(0, 77)}…`;
+}