diff --git a/packages/prosemirror-markdown/test/generative/attr-arbitraries.ts b/packages/prosemirror-markdown/test/generative/attr-arbitraries.ts
index b0f88fd4..c3c9a03d 100644
--- a/packages/prosemirror-markdown/test/generative/attr-arbitraries.ts
+++ b/packages/prosemirror-markdown/test/generative/attr-arbitraries.ts
@@ -120,6 +120,11 @@ interface AttrPolicy {
 const num = (...xs: number[]) => fc.constantFrom(...xs);
 const str = (...xs: string[]) => fc.constantFrom(...xs);
 const widthStr = str('120', '320', '640');
+// Media `aspectRatio`/`size` are stringified numerics too (converter emits
+// String(value)); the schema parseHTML reads them back as strings. Fuzz as
+// plausible numeric strings so they round-trip byte-stably like widthStr.
+const aspectRatioStr = str('1.5', '0.75', '1');
+const sizeStr = str('320', '640');
 
 // The documented override table, keyed `type.attr`. Every entry is grounded in
 // the empirical converter probe (see flat-roundtrip.property.test.ts header).
@@ -149,16 +154,57 @@ const OVERRIDES: Record<string, AttrPolicy> = {
   'image.title': { arb: letterPhraseArb },
   'image.width': { arb: widthStr, degen: '' },
   'image.height': { arb: widthStr, degen: '' },
+  // #351 (this PR): media sizing/family attrs ride in the `<!--img {…}-->`
+  // comment JSON via String(value); parseHTML reads them back as strings, so a
+  // numeric string round-trips byte-stably (mirrors image.width).
+  'image.size': { arb: sizeStr, degen: '' },
+  'image.aspectRatio': { arb: aspectRatioStr },
+  // caption is text carried verbatim in the comment JSON (mirrors image.alt).
+  'image.caption': { arb: letterPhraseArb, degen: '' },
   'video.src': { noDefault: true, arb: urlArb, degen: '' },
   'video.alt': { arb: letterPhraseArb },
   'video.width': { arb: widthStr },
   'video.height': { arb: widthStr },
+  // #351 (this PR): video sizing/align ride in the `<!--video {…}-->` comment.
+  'video.size': { arb: sizeStr },
+  'video.aspectRatio': { arb: aspectRatioStr },
+  // align default "center" is dropped on export; fuzz non-center only.
+  'video.align': { arb: str('left', 'right') },
   'audio.src': { noDefault: true, arb: urlArb, degen: '' },
   'youtube.src': { noDefault: true, arb: urlArb },
+  // #351 (this PR): youtube width/height/align ride in the `<!--youtube {…}-->`
+  // comment JSON (String()-coerced dimensions, non-center align only).
+  'youtube.width': { arb: widthStr },
+  'youtube.height': { arb: widthStr },
+  'youtube.align': { arb: str('left', 'right') },
   'pdf.src': { noDefault: true, arb: urlArb },
   'pdf.name': { arb: phraseArb },
+  // #351 (this PR): pdf size/width/height ride in the `<!--pdf {…}-->` comment
+  // (String()-coerced dimensions, read back as strings).
+  'pdf.size': { arb: sizeStr },
+  'pdf.width': { arb: widthStr },
+  'pdf.height': { arb: widthStr },
   'drawio.src': { noDefault: true, arb: urlArb },
+  // #351 (this PR): drawio family attrs ride in the `<!--drawio {…}-->` comment.
+  // Dimensions/size/aspectRatio are String()-coerced numeric strings; title/alt
+  // are text carried verbatim; align fuzzed non-center only.
+  'drawio.width': { arb: widthStr },
+  'drawio.height': { arb: widthStr },
+  'drawio.size': { arb: sizeStr },
+  'drawio.aspectRatio': { arb: aspectRatioStr },
+  'drawio.align': { arb: str('left', 'right') },
+  'drawio.title': { arb: letterPhraseArb },
+  'drawio.alt': { arb: letterPhraseArb },
   'excalidraw.src': { noDefault: true, arb: urlArb },
+  // #351 (this PR): excalidraw family attrs ride in the `<!--excalidraw {…}-->`
+  // comment (same shape as the drawio family above).
+  'excalidraw.width': { arb: widthStr },
+  'excalidraw.height': { arb: widthStr },
+  'excalidraw.size': { arb: sizeStr },
+  'excalidraw.aspectRatio': { arb: aspectRatioStr },
+  'excalidraw.align': { arb: str('left', 'right') },
+  'excalidraw.title': { arb: letterPhraseArb },
+  'excalidraw.alt': { arb: letterPhraseArb },
   'attachment.url': { noDefault: true, arb: urlArb },
   'attachment.name': { arb: phraseArb },
   // ── callout / status ─────────────────────────────────────────────────────
@@ -201,8 +247,19 @@ const OVERRIDES: Record<string, AttrPolicy> = {
   // ── embed (schema keeps width/height NUMERIC, not string-coerced) ─────────
   'embed.src': { noDefault: true, arb: urlArb, degen: '' },
   'embed.provider': { noDefault: true, arb: str('iframe', 'youtube', 'vimeo') },
-  'embed.width': { always: true, frozen: true },
-  'embed.height': { always: true, frozen: true },
+  // #351 (this PR): the embed schema defaults width/height to the NUMBERS 800/600
+  // and the converter only emits them when they differ. But the value round-trips
+  // as a STRING: export stringifies into the comment JSON (String(width)) and the
+  // import path (embedToHtml -> data-width -> embed parseHTML) reads it back as a
+  // string, so an authored NUMBER 400 would diverge under P1 (400 vs "400"). Fuzz
+  // as numeric STRINGS avoiding "800"/"600" so they round-trip byte-stably. The
+  // 800/600 numeric default state still round-trips (omitted on export, re-
+  // materialized as the numeric default). `always` stays because these are
+  // materialized on import but absent from canonicalize's KNOWN_DEFAULTS.
+  'embed.width': { always: true, arb: str('400', '1000', '1200') },
+  'embed.height': { always: true, arb: str('300', '500', '900') },
+  // align default "center" is dropped on export; fuzz non-center only.
+  'embed.align': { arb: str('left', 'right') },
   // ── subpages / math / htmlEmbed ──────────────────────────────────────────
   'subpages.recursive': { always: true, arb: fc.constant(true) }, // boolean, default false
   'mathBlock.text': { noDefault: true, arb: str('x^2', 'a < b', '\\frac{1}{2}'), degen: '' },
diff --git a/packages/prosemirror-markdown/test/generative/flat-roundtrip.property.test.ts b/packages/prosemirror-markdown/test/generative/flat-roundtrip.property.test.ts
index 5be0631c..dfdd2b8c 100644
--- a/packages/prosemirror-markdown/test/generative/flat-roundtrip.property.test.ts
+++ b/packages/prosemirror-markdown/test/generative/flat-roundtrip.property.test.ts
@@ -28,32 +28,32 @@ import {
 // a NEW attribute (or a newly-frozen one) that lands in this bucket flips the
 // snapshot test red and forces a reviewer to classify it. Each belongs to one of:
 //   - internal/opaque ids & placeholders (attachmentId, slugId, placeholder,
-//     creatorId, anchorId) — no meaningful non-default to assert;
-//   - dimensions/among the media family with no standalone md form here
-//     (aspectRatio, size, caption, drawio/excalidraw/pdf/video/youtube w/h/align)
-//     — round-trip candidates deferred to a later PR, not silently dropped;
+//     creatorId, anchorId, mime) — no meaningful non-default to assert. These stay
+//     frozen: their value is an opaque token carried verbatim, not a round-trip
+//     shape worth fuzzing;
 //   - ACCEPTED limitations with no md representation (indent, callout.icon,
 //     orderedList.type, table spans/bg/colwidth).
+// The media dimension/family attrs (image/video/youtube/drawio/excalidraw/pdf/
+// embed width/height/align/size/aspectRatio/caption/title/alt) that were
+// previously "deferred to a later PR" are IMPLEMENTED (value-fuzzed) in THIS PR
+// via the OVERRIDES table in attr-arbitraries.ts — they ride in the discriminator
+// comment JSON and round-trip byte-stably, so they are no longer allowlisted.
 const ATTR_VALUE_FUZZ_ALLOWLIST = new Set<string>([
   'attachment.attachmentId', 'attachment.mime', 'attachment.placeholder', 'attachment.size',
   'audio.attachmentId', 'audio.placeholder', 'audio.size',
   'callout.icon',
-  'drawio.align', 'drawio.alt', 'drawio.aspectRatio', 'drawio.attachmentId',
-  'drawio.height', 'drawio.size', 'drawio.title', 'drawio.width',
-  'embed.align', 'embed.height', 'embed.width',
-  'excalidraw.align', 'excalidraw.alt', 'excalidraw.aspectRatio', 'excalidraw.attachmentId',
-  'excalidraw.height', 'excalidraw.size', 'excalidraw.title', 'excalidraw.width',
+  'drawio.attachmentId',
+  'excalidraw.attachmentId',
   'heading.indent',
-  'image.aspectRatio', 'image.attachmentId', 'image.caption', 'image.placeholder', 'image.size',
+  'image.attachmentId', 'image.placeholder',
   'mention.anchorId', 'mention.creatorId', 'mention.slugId',
   'orderedList.type', 'paragraph.indent',
-  'pdf.attachmentId', 'pdf.height', 'pdf.placeholder', 'pdf.size', 'pdf.width',
+  'pdf.attachmentId', 'pdf.placeholder',
   'tableCell.backgroundColor', 'tableCell.backgroundColorName', 'tableCell.colspan',
   'tableCell.colwidth', 'tableCell.rowspan',
   'tableHeader.backgroundColor', 'tableHeader.backgroundColorName', 'tableHeader.colspan',
   'tableHeader.colwidth', 'tableHeader.rowspan',
-  'video.align', 'video.aspectRatio', 'video.attachmentId', 'video.placeholder', 'video.size',
-  'youtube.align', 'youtube.height', 'youtube.width',
+  'video.attachmentId', 'video.placeholder',
 ]);
 
 // ── MARK attribute-value coverage ───────────────────────────────────────────