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

src/platforms/reddit.ts

@@ -104,6 +104,10 @@ function fromRedditVideo(video: RedditVideo, poster: string | undefined): Media[
   const common = {
     ...sized(video.width, video.height),
     ...(video.duration ? { durationSec: video.duration } : {}),
+    // The still belongs to the video and not to the format it is served in.
+    // The HLS branch used to drop it, which is why a post with sound showed
+    // an empty box where every silent one showed a frame.
+    ...(poster ? { poster: { url: poster } } : {}),
   };
 
   if (video.has_audio !== false && video.hls_url) {
@@ -114,12 +118,7 @@ function fromRedditVideo(video: RedditVideo, poster: string | undefined): Media[
   if (url === video.hls_url) {
     return [{ kind: 'video', url, hls: true, direct: true, ...common }];
   }
-  return [{
-    kind: 'video',
-    url,
-    ...(poster ? { poster: { url: poster } } : {}),
-    ...common,
-  }];
+  return [{ kind: 'video', url, ...common }];
 }
 
 /** One entry of a gallery post. Reddit keeps the pictures somewhere other

src/render/post.ts

@@ -14,11 +14,29 @@ function aspect(item: Media): string {
   return item.width && item.height ? `aspect-ratio: ${item.width} / ${item.height};` : '';
 }
 
+/**
+ * A stand-in poster carrying nothing but the video's shape.
+ *
+ * The ratio above is not enough on its own before the video has any data: a
+ * `<video>` in that state has a natural size of 300x150, and WebKit sizes it
+ * from that rather than from the ratio, so a portrait video sat in a squat
+ * landscape box until you pressed play and it snapped to shape. A video's
+ * size before its data arrives is its poster's, which makes an empty SVG of
+ * the right shape enough to put the box right, and as a data URI it costs no
+ * request. Only for a video the platform gave no poster for, since a real one
+ * already says the same thing.
+ */
+function placeholderPoster(item: Media): string | undefined {
+  if (!item.width || !item.height) return undefined;
+  const svg = `<svg xmlns="http://www.w3.org/2000/svg" width="${item.width}" height="${item.height}"/>`;
+  return `data:image/svg+xml,${encodeURIComponent(svg)}`;
+}
+
 function renderItem(item: Media, index: number): Raw {
   const src = proxyUrlFor(item);
 
   if (item.kind === 'video') {
-    const poster = item.poster ? proxyUrlFor(item.poster) : undefined;
+    const poster = item.poster ? proxyUrlFor(item.poster) : placeholderPoster(item);
     return html`<figure class="item item--video" data-index="${index}">
       <video
         controls