diff --git a/packages/blocks-engine/src/__fixtures__/cases.json b/packages/blocks-engine/src/__fixtures__/cases.json
index bab4edac9..4f26aa16b 100644
--- a/packages/blocks-engine/src/__fixtures__/cases.json
+++ b/packages/blocks-engine/src/__fixtures__/cases.json
@@ -109,13 +109,13 @@
     "id": "compose.generic-details",
     "op": "compose",
     "input": "<details><summary>Q</summary><p>A</p></details>",
-    "expected": "<!-- wp:details -->\n<details class=\"wp-block-details\"><summary>Q</summary><p>A</p></details>\n<!-- /wp:details -->"
+    "expected": "<!-- wp:details -->\n<details class=\"wp-block-details\"><summary>Q</summary>\n<!-- wp:html {\"metadata\":{\"name\":\"lib-coverage-island\"}} -->\n<p>A</p>\n<!-- /wp:html -->\n</details>\n<!-- /wp:details -->"
   },
   {
     "id": "compose.generic-callout",
     "op": "compose",
     "input": "<div class=\"callout\"><h3>Note</h3><p>body</p></div>",
-    "expected": "<!-- wp:group {\"className\":\"wp-block-group is-style-callout\",\"layout\":{\"type\":\"constrained\"}} -->\n<div class=\"wp-block-group is-style-callout\"><h3>Note</h3><p>body</p></div>\n<!-- /wp:group -->"
+    "expected": "<!-- wp:group {\"className\":\"callout\",\"layout\":{\"type\":\"constrained\"}} -->\n<div class=\"wp-block-group callout\">\n<!-- wp:html {\"metadata\":{\"name\":\"lib-coverage-island\"}} -->\n<h3>Note</h3><p>body</p>\n<!-- /wp:html -->\n</div>\n<!-- /wp:group -->"
   },
   {
     "id": "compose.generic-pullquote",
@@ -133,7 +133,7 @@
     "id": "compose.generic-media-text",
     "op": "compose",
     "input": "<div class=\"media-text\"><figure><img src=\"https://cdn.test/a.jpg\" alt=\"A\"/></figure><div><h3>Title</h3><p>copy</p></div></div>",
-    "expected": "<!-- wp:media-text {\"mediaType\":\"image\"} -->\n<div class=\"wp-block-media-text is-stacked-on-mobile\"><figure class=\"wp-block-media-text__media\"><img src=\"https://cdn.test/a.jpg\" alt=\"A\"/></figure><div class=\"wp-block-media-text__content\"><h3>Title</h3><p>copy</p></div></div>\n<!-- /wp:media-text -->"
+    "expected": "<!-- wp:media-text {\"mediaType\":\"image\"} -->\n<div class=\"wp-block-media-text is-stacked-on-mobile\"><figure class=\"wp-block-media-text__media\"><img src=\"https://cdn.test/a.jpg\" alt=\"A\"/></figure><div class=\"wp-block-media-text__content\"><!-- wp:html {\"metadata\":{\"name\":\"lib-coverage-island\"}} -->\n<h3>Title</h3><p>copy</p>\n<!-- /wp:html --></div></div>\n<!-- /wp:media-text -->"
   },
   {
     "id": "compose.sample-youtube-iframe",
@@ -145,7 +145,7 @@
     "id": "compose.sample-callout-div",
     "op": "compose",
     "input": "<div class=\"notice callout\"><h3>Shipping pause</h3><p>Orders resume Monday.</p></div>",
-    "expected": "<!-- wp:group {\"className\":\"wp-block-group is-style-notice\",\"layout\":{\"type\":\"constrained\"}} -->\n<div class=\"wp-block-group is-style-notice\"><h3>Shipping pause</h3><p>Orders resume Monday.</p></div>\n<!-- /wp:group -->"
+    "expected": "<!-- wp:group {\"className\":\"notice callout\",\"layout\":{\"type\":\"constrained\"}} -->\n<div class=\"wp-block-group notice callout\">\n<!-- wp:html {\"metadata\":{\"name\":\"lib-coverage-island\"}} -->\n<h3>Shipping pause</h3><p>Orders resume Monday.</p>\n<!-- /wp:html -->\n</div>\n<!-- /wp:group -->"
   },
   {
     "id": "compose.sample-semantic-section-fallback",
diff --git a/packages/blocks-engine/src/__tests__/content-preservation.test.ts b/packages/blocks-engine/src/__tests__/content-preservation.test.ts
new file mode 100644
index 000000000..04efea3db
--- /dev/null
+++ b/packages/blocks-engine/src/__tests__/content-preservation.test.ts
@@ -0,0 +1,108 @@
+import { afterAll, describe, expect, it } from 'vitest';
+
+import cases from '../__fixtures__/cases.json' with { type: 'json' };
+import { convertReport } from '../index.js';
+import { createWorker } from '../pool/pool.js';
+import { canonicalize } from '../wp/canonicalize';
+
+type Fixture = { id: string; op: string; input: string };
+
+// What a reader of the page sees: its words, and the media it loads (a
+// source may become an embed URL, so any occurrence of it counts).
+function words(html: string): string[] {
+  return html
+    .replace(/<!--[\s\S]*?-->/gu, ' ')
+    .replace(/<[^>]+>/gu, ' ')
+    .split(/\s+/u)
+    .filter(Boolean);
+}
+
+function missing(input: string, output: string): string[] {
+  const left = words(output);
+  const lostWords = words(input).filter((word) => {
+    const at = left.indexOf(word);
+    if (at === -1) return true;
+    left.splice(at, 1);
+    return false;
+  });
+  const lostSources = Array.from(input.matchAll(/\ssrc="([^"]+)"/gu), (match) => match[1]).filter(
+    (source) => !output.includes(source),
+  );
+  return [...lostWords, ...lostSources];
+}
+
+const pool = createWorker({ size: 1 });
+afterAll(() => pool.stop());
+
+const card =
+  '<div class="paw-card"><img src="https://example.com/a.png" alt="A"><h3>Coco Rope Tug</h3><p>Braided cotton rope.</p></div>';
+
+const inputs: Array<[string, string]> = [
+  ['a card', card],
+  ['a card with a price and a link', '<div class="paw-card"><h3>Tug</h3><p class="price">$12</p><a class="btn" href="/x">Add</a></div>'],
+  ['cards in a grid', `<div class="card-grid">${card}${card}</div>`],
+  ['a callout', '<div class="callout"><p>Heads up.</p></div>'],
+  ['an accordion item', '<div class="faq-item"><h3>Shipping?</h3><div><p>Two days.</p></div></div>'],
+  ...(cases as Fixture[])
+    .filter((fixture) => fixture.op === 'compose')
+    .map((fixture): [string, string] => [fixture.id, fixture.input]),
+];
+
+describe('conversion keeps what the page shows', () => {
+  it.each(inputs)('%s', async (_name, html) => {
+    const report = await convertReport(html, { url: 'https://x.test/' }, { pool });
+
+    expect(missing(html, report.blockMarkup)).toEqual([]);
+    // Converted blocks hold their content themselves; none rely on the
+    // canonicalize guard keeping an invalid block.
+    expect(report.diagnostics.map((finding) => finding.code)).not.toContain('invalid_block_preserved');
+  });
+
+  it('keeps a card class the site styles and invents no block style', async () => {
+    const report = await convertReport(card, { url: 'https://x.test/' }, { pool });
+
+    expect(report.blockMarkup).toContain('paw-card');
+    expect(report.blockMarkup).not.toContain('is-style-');
+    expect(report.status).toBe('success_with_warnings');
+  });
+});
+
+describe('canonicalize keeps an invalid block whose re-save drops content', () => {
+  const lossy: Array<[string, string, string[]]> = [
+    [
+      'a group holding raw HTML',
+      '<!-- wp:group -->\n<div class="wp-block-group"><p>Kept words</p><img src="https://example.com/k.png" alt=""></div>\n<!-- /wp:group -->',
+      ['core/group'],
+    ],
+    [
+      'the same group inside columns',
+      '<!-- wp:columns -->\n<div class="wp-block-columns"><!-- wp:column -->\n<div class="wp-block-column"><!-- wp:group -->\n<div class="wp-block-group"><p>Kept words</p></div>\n<!-- /wp:group --></div>\n<!-- /wp:column --></div>\n<!-- /wp:columns -->',
+      ['core/group'],
+    ],
+  ];
+
+  it.each(lossy)('%s', (_name, markup, preserved) => {
+    const result = canonicalize(markup);
+
+    expect(missing(markup, result.html)).toEqual([]);
+    expect(result.preservedBlocks).toEqual(preserved);
+  });
+
+  it('still fixes an invalid block whose re-save keeps its content', () => {
+    const markup = '<!-- wp:paragraph {"className":"lead"} -->\n<p>Hello there</p>\n<!-- /wp:paragraph -->';
+    const result = canonicalize(markup);
+
+    expect(result.html).toContain('<p class="lead">Hello there</p>');
+    expect(result.preservedBlocks).toEqual([]);
+  });
+
+  it('reports a kept block as a warning', async () => {
+    const markup = lossy[0][1];
+    const report = await convertReport(markup, { url: '' }, { pool });
+
+    expect(report.status).toBe('success_with_warnings');
+    expect(report.diagnostics).toContainEqual(
+      expect.objectContaining({ code: 'invalid_block_preserved', severity: 'warning', selector: 'core/group' }),
+    );
+  });
+});
diff --git a/packages/blocks-engine/src/__tests__/pool.contract.test.ts b/packages/blocks-engine/src/__tests__/pool.contract.test.ts
index 89157be70..a66701b39 100644
--- a/packages/blocks-engine/src/__tests__/pool.contract.test.ts
+++ b/packages/blocks-engine/src/__tests__/pool.contract.test.ts
@@ -58,6 +58,7 @@ describe('worker pool contract', () => {
       }[];
       htmlIslandDistinctCount?: number;
       htmlIslandOccurrencesTruncated?: boolean;
+      preservedBlocks?: string[];
       degraded: boolean;
     }>();
 
diff --git a/packages/blocks-engine/src/catalog.ts b/packages/blocks-engine/src/catalog.ts
index e9120af4e..37c6a8d6a 100644
--- a/packages/blocks-engine/src/catalog.ts
+++ b/packages/blocks-engine/src/catalog.ts
@@ -130,10 +130,11 @@ function recurseInner(
   ctx: ConversionContext,
   htmlFallback: HtmlFallbackEmitter,
 ): string {
-  const nested = genericHtmlToBlocks(html, ctx, htmlFallback);
-  if (nested && nested.trim()) return `\n${nested}\n`;
-  const clean = sanitize(html).trim();
-  return clean ? clean : '';
+  if (!sanitize(html).trim()) return '';
+  // Inner content is always block markup: a container block saves only its
+  // inner blocks, so raw HTML inside it is discarded on the editor's re-save.
+  const nested = genericHtmlToBlocks(html, ctx, htmlFallback) ?? htmlFallback(html);
+  return `\n${nested}\n`;
 }
 
 const HEADING_TAGS = new Set(['h1', 'h2', 'h3', 'h4', 'h5', 'h6']);
@@ -189,11 +190,11 @@ function tryCallout(
   if (!CALLOUT_RE.test(cls)) return null;
   const inner = recurseInner($el.html() ?? '', ctx, htmlFallback);
   if (!inner.trim()) return null;
-  const matchedClass = (cls.match(CALLOUT_RE) || [])[0] ?? 'callout';
-  const className = `wp-block-group is-style-${matchedClass}`;
+  // The authored classes are what the site's CSS targets; keep them as is.
+  const className = cls.split(/\s+/).filter((name) => name && name !== 'wp-block-group').join(' ');
   return (
     `<!-- wp:group {"className":${JSON.stringify(className)},"layout":{"type":"constrained"}} -->\n` +
-    `<div class="${escapeAttr(className)}">${inner}</div>\n` +
+    `<div class="${escapeAttr(`wp-block-group ${className}`)}">${inner}</div>\n` +
     `<!-- /wp:group -->`
   );
 }
diff --git a/packages/blocks-engine/src/pool/types.ts b/packages/blocks-engine/src/pool/types.ts
index 47d129cf9..2eef18f60 100644
--- a/packages/blocks-engine/src/pool/types.ts
+++ b/packages/blocks-engine/src/pool/types.ts
@@ -21,6 +21,7 @@ export interface FixResult {
   htmlIslandOccurrences?: HtmlIslandOccurrence[];
   htmlIslandDistinctCount?: number;
   htmlIslandOccurrencesTruncated?: boolean;
+  preservedBlocks?: string[];
   degraded: boolean;
 }
 
diff --git a/packages/blocks-engine/src/report/findings.ts b/packages/blocks-engine/src/report/findings.ts
index d369a3b83..afe5d45e3 100644
--- a/packages/blocks-engine/src/report/findings.ts
+++ b/packages/blocks-engine/src/report/findings.ts
@@ -267,6 +267,15 @@ export function buildReport({
     message: issue,
   }));
 
+  for (const name of fixResult.preservedBlocks ?? []) {
+    diagnostics.push({
+      code: 'invalid_block_preserved',
+      severity: 'warning',
+      message: `${name} kept as written: the editor's re-save would drop content it shows.`,
+      selector: name,
+    });
+  }
+
   if (fixResult.degraded) {
     diagnostics.push({
       code: 'conversion_degraded',
diff --git a/packages/blocks-engine/src/report/schema.ts b/packages/blocks-engine/src/report/schema.ts
index 35f75fc60..ee717c7f3 100644
--- a/packages/blocks-engine/src/report/schema.ts
+++ b/packages/blocks-engine/src/report/schema.ts
@@ -23,6 +23,7 @@ export const CONVERSION_FINDING_CODES = [
   'conversion_degraded',
   'fallback_inventory_truncated',
   'content_dropped',
+  'invalid_block_preserved',
   'hero_image_layering_risk',
   'body_text_promoted_to_heading',
   'heading_inside_list_item',
diff --git a/packages/blocks-engine/src/wp/canonicalize.ts b/packages/blocks-engine/src/wp/canonicalize.ts
index f50573e00..8565e9730 100644
--- a/packages/blocks-engine/src/wp/canonicalize.ts
+++ b/packages/blocks-engine/src/wp/canonicalize.ts
@@ -27,6 +27,8 @@ export type CanonicalizeResult = {
   htmlIslandOccurrences: HtmlIslandOccurrence[];
   htmlIslandDistinctCount: number;
   htmlIslandOccurrencesTruncated: boolean;
+  /** Invalid blocks kept as written because their re-save drops content. */
+  preservedBlocks: string[];
   degraded: boolean;
 };
 
@@ -271,12 +273,48 @@ function commentAttrsAltered(
 // attributes (content etc.) never appear in comment attrs, so author intent
 // can't clobber them; createBlock() drops keys unknown to the current schema.
 //
+// The words and media sources a reader sees in serialized markup, counted.
+function visibleContent(markup: string): Map<string, number> {
+  const counts = new Map<string, number>();
+  const sources = Array.from(markup.matchAll(/\ssrc="([^"]+)"/gu), (match) => `src:${match[1]}`);
+  const words = markup
+    .replace(/<!--[\s\S]*?-->/gu, ' ')
+    .replace(/<[^>]+>/gu, ' ')
+    .split(/\s+/u)
+    .filter(Boolean);
+  for (const item of [...sources, ...words]) counts.set(item, (counts.get(item) ?? 0) + 1);
+  return counts;
+}
+
+function dropsContent(before: string, after: string): boolean {
+  const kept = visibleContent(after);
+  for (const [item, count] of visibleContent(before)) {
+    if ((kept.get(item) ?? 0) < count) return true;
+  }
+  return false;
+}
+
+type BlockFixer = {
+  getBlockAttributes: WordpressBlocks['getBlockAttributes'];
+  createBlock: WordpressBlocks['createBlock'];
+  serialize: WordpressBlocks['serialize'];
+  preserved: string[];
+};
+
+//
+// An invalid block's re-save is rebuilt from its attributes and inner
+// blocks, so anything else its markup held is lost (a container's raw inner
+// HTML, for one). When that re-save would drop words or media the block
+// showed, the block is kept as written: serialize() emits an invalid block's
+// original source, which is what the editor itself does until a person
+// chooses to recover it.
+//
 function fixBlockRecursively(
   block: ParsedBlock,
   rawBlock: RawBlock | undefined,
-  getBlockAttributes: WordpressBlocks['getBlockAttributes'],
-  createBlock: WordpressBlocks['createBlock'],
+  fixer: BlockFixer,
 ): { block: ParsedBlock; wasFixed: boolean } {
+  const { getBlockAttributes, createBlock, serialize } = fixer;
   const fixedInnerBlocks = [];
 
   if (block.innerBlocks && block.innerBlocks.length > 0) {
@@ -295,12 +333,7 @@ function fixBlockRecursively(
       }
       const rawInnerBlock =
         rawIndex < rawInner.length ? rawInner[rawIndex++] : undefined;
-      const result = fixBlockRecursively(
-        innerBlock,
-        rawInnerBlock,
-        getBlockAttributes,
-        createBlock,
-      );
+      const result = fixBlockRecursively(innerBlock, rawInnerBlock, fixer);
       fixedInnerBlocks.push(result.block);
     }
   }
@@ -328,6 +361,11 @@ function fixBlockRecursively(
     fixedInnerBlocks.length > 0 ? fixedInnerBlocks : undefined,
   );
 
+  if (block.isValid === false && dropsContent(serialize([block]), serialize([fixedBlock]))) {
+    fixer.preserved.push(block.name);
+    return { block, wasFixed: false };
+  }
+
   return { block: fixedBlock, wasFixed: true };
 }
 
@@ -464,6 +502,7 @@ export function canonicalize(markup: string): CanonicalizeResult {
     const fixedIssues: string[] = [];
     collectIssues(blocks, fixedIssues);
 
+    const fixer: BlockFixer = { getBlockAttributes, createBlock, serialize, preserved: [] };
     let rawIndex = 0;
     const fixedBlocks = blocks.map((block) => {
       while (
@@ -474,12 +513,7 @@ export function canonicalize(markup: string): CanonicalizeResult {
       }
       const rawBlock =
         rawIndex < rawBlocks.length ? rawBlocks[rawIndex++] : undefined;
-      return fixBlockRecursively(
-        block,
-        rawBlock,
-        getBlockAttributes,
-        createBlock,
-      ).block;
+      return fixBlockRecursively(block, rawBlock, fixer).block;
     });
 
     let fixedHtml = serialize(fixedBlocks);
@@ -512,6 +546,7 @@ export function canonicalize(markup: string): CanonicalizeResult {
       changed: wasChanged,
       fixedIssues,
       ...inventory,
+      preservedBlocks: fixer.preserved,
       degraded: false,
     };
   } catch (error) {
@@ -527,6 +562,7 @@ export function canonicalize(markup: string): CanonicalizeResult {
       htmlIslandOccurrences: [],
       htmlIslandDistinctCount: 0,
       htmlIslandOccurrencesTruncated: false,
+      preservedBlocks: [],
       degraded: true,
     };
   }