Video

A video is an image whose file ends in .mp4, .m4v or .webm:

![video](clip.mp4)
![The demo](clip.mp4#t=12.5)
![demo loop muted](clip.mp4)

Syntax

The file is read from the deck’s folder, or from an http or https URL. An absolute path, a path that leaves the folder or any other scheme is refused. #t=12.5, or #t=1:23 for minutes and seconds, picks the poster frame: the picture shown before playback and by the outputs that cannot play the video. The media-fragment form #t=10,20 is accepted too, and its start is used. Without it, the poster is the first frame.

Keywords in the alternative text, written like Marp image keywords but go-slides’ own, set how the video plays: autoplay, loop and muted. autoplay is honoured only together with muted; otherwise it is ignored, with a warning. The other words of the alternative text are the accessible label.

What each output shows

• HTML (the page, and the playground’s preview): a <video controls playsinline preload="metadata" poster=...> element. The poster is the decoded frame, embedded as a JPEG data URI. A video never plays with sound by itself. When the browser asks for less motion (prefers-reduced-motion), autoplay and loop are removed and the video is paused. A video that autoplays (muted) plays only while at least half of it is visible, and pauses when its slide is not shown.
• PDF (--pdf): the poster frame as a picture, with a link to the video file: the relative path as written in the deck, or the URL for an http or https source. Playback happens only by opening the link. The PDF carries no RichMedia annotation: it would play only in Acrobat, and is deliberately not used.
• PowerPoint (--pptx): the video is embedded as a media part (ppt/media/slidesvideoN.mp4|webm) with its poster, as a:videoFile plus the p14:media extension. Keynote (macOS): imports it as a movie and plays it (verified). LibreOffice Impress (macOS): imports PowerPoint and OpenDocument files as a video and decodes it (verified). PowerPoint: not yet verified. The autoplay, loop and muted keywords are not carried into PowerPoint or OpenDocument.
• OpenDocument (--odp): the video is embedded under Video/ as a draw:plugin (ODF 1.3 Part 3, 10.4.8), before the poster draw:image, and listed in the manifest. LibreOffice Impress imports it as a video and decodes it (verified); live slideshow playback depends on its media backend and the codec.
• URL sources: the HTML page streams the video from the URL. The exports show a neutral card linked to the URL. Nothing is fetched at build time, and nothing is embedded in PowerPoint or OpenDocument.

Containers and posters

MP4 (ISO BMFF) and WebM (Matroska) files are read, in Go; ffmpeg is never run. The poster is decoded for H.264 only (avc1 or avc3), the one video decoder of go-avkit. Other codecs (H.265, VP8, VP9, AV1) still play in HTML and are embedded in PowerPoint and OpenDocument, but their poster is a neutral dark card with a play triangle, and the warning video.codec is printed.

Limits

A video is untrusted input, so it is bounded: the file (64 MiB), the boxes or elements (262,144) and their nesting (16 levels), the frame (4096 by 4096) and the poster decoding (at most 600 samples from the preceding keyframe, 8 MiB per sample). A poster too far from a keyframe gets the card and the warning video.limit.

A refused video is replaced in the slide by a note, [video not shown: ...]; the build goes on, and the problem is a warning with a code:

• video.syntax: not an MP4 or WebM file, no video track, or an invalid time;
• video.limit: a bound above is exceeded;
• video.codec: the poster cannot be decoded for this codec;
• video.path: an absolute path, a path that leaves the deck’s folder, or a scheme other than http and https;
• file.missing: the file does not exist.

In the playground

Once released, a video can be added to the project like the other files. Its bytes stay in the browser, as blob URLs, and are never uploaded. Try it in the playground.