Give a video its shape before it has any data

A Reddit video sat in the wrong box until you pressed play, for two reasons
that looked like one.

The renderer put only `aspect-ratio` on the `<video>`. A video with no data
has a natural size of 300x150, and WebKit sizes a replaced element from that
rather than from the ratio, so a portrait video got a squat landscape box and
kept it until playback supplied real dimensions. Chromium stretch-fits
instead and gets it right, which is why this only showed on Safari. A
video's size before its data arrives is its poster's, so one with no poster
of its own now gets an empty SVG of the right shape as a stand-in: a data
URI, so it costs no request. Measured in both engines across portrait,
landscape, square and small.

And Reddit's videos with sound had no poster to be sized by, because
`fromRedditVideo` attached the still from `preview.images` to the MP4 branch
alone. The still belongs to the video, not to the format it is served in, so
every post with sound showed an empty box where a silent one showed a frame.
It now goes on both.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01PLkmgp1fWbA4XbarxKdRKt

test/render.test.ts

@@ -90,6 +90,27 @@ test('a video gets native controls and a source, not an iframe', () => {
   assert.ok(!page.includes('<iframe'));
 });
 
+test('a video with no poster still carries its shape, so the box is right before play', () => {
+  // WebKit sizes a video from its natural size, which without data or a
+  // poster is 300x150 — a portrait video sat in a landscape box until you
+  // pressed play. The poster is where the shape comes from until then.
+  const page = renderPost(withMedia([
+    { kind: 'video', url: 'https://video.example/p.m3u8', hls: true, direct: true, width: 720, height: 1280 },
+  ]));
+  assert.ok(page.includes('aspect-ratio: 720 / 1280;'));
+  assert.match(page, /poster="data:image\/svg\+xml,[^"]*width%3D%22720%22[^"]*height%3D%221280%22/);
+});
+
+test('a real poster is left in place, and an unmeasured video gets none', () => {
+  const withPoster = renderPost(withMedia([
+    { kind: 'video', url: 'https://cdn/v.mp4', width: 720, height: 1280, poster: { url: 'https://cdn/p.jpg' } },
+  ]));
+  assert.match(withPoster, /poster="\/m\//);
+  assert.ok(!withPoster.includes('data:image/svg'));
+
+  assert.ok(!renderPost(withMedia([{ kind: 'video', url: 'https://cdn/v.mp4' }])).includes('poster='));
+});
+
 test('the copy button carries the clean original URL, and so does the page text', () => {
   const page = renderPost(post());
   assert.ok(page.includes('data-url="https://www.instagram.com/p/ABC/"'));