v0.1.12513 Aug 2026·Web APIs & DOM, Media

fMP4 / MP4 streaming: the engine parses the container the web actually serves

Media Source Extensions - how the web streams adaptive video - has worked in Floati for WebM for a while. But the format almost every real streaming site (a video site, DASH, HLS) actually serves is fragmented MP4 (ISO-BMFF), and that was completely unsupported. This release builds a pure-Rust fMP4 demuxer - no dependencies, no unsafe code, no decode - and wires it into the existing MSE machinery: an init segment now parses to its track list (codec, dimensions, timescale), and each media fragment parses to its exact buffered time range from box metadata alone. Fed a real fragmented MP4, a SourceBuffer reports the same precise, multi-range, out-of-order-tolerant buffered coverage it already does for WebM. This is the container half of MP4 streaming; the codec half (H.264/AV1/AAC decode) is deliberately separate and gated behind a decoder decision, so MP4 is not yet advertised as playable - the engine never tells a page it can play what it cannot yet decode. Media moves 39 to 40; overall holds at 52.

Parity with the reference browser

74overall, as of v0.1.343

Web APIs / DOM surface42

Security architecture66

GPU / graphics acceleration68

DevTools / extensions72

Networking85

Media85

Accessibility85

Real-world site compatibility99

HTML parsing100

CSS parsing + selectors100

Layout: block / flex / grid / tables / writing-modes100

Paint + visual100

JavaScript language100

JavaScript performance (JIT)100

HTML parsing100

What changed 3 changes

Media	A pure-Rust ISO-BMFF / fragmented-MP4 box demuxer (core-demux mp4.rs): reads the box tree (32-bit and 64-bit sizes, full-box versions), parses the init segment (ftyp+moov) into a track list - handler kind, sample-entry codec 4CC, 16.16 fixed-point dimensions, media timescale - and parses each media fragment (moof: tfhd/tfdt/trun) into its exact time coverage from metadata alone, with no codec and no sample decode. Verified against a real fragmented MP4 fixture down to the millisecond.
Media	Wired into Media Source Extensions: a SourceBuffer sniffs its container on the first append and routes fMP4 through the new demuxer into the same container-neutral segment store WebM already uses, so buffered TimeRanges, multi-range gaps, out-of-order and overlapping (adaptive-bitrate) appends, duration and video dimensions all work for MP4 exactly as for WebM. Proven by direct SourceBuffer tests: two contiguous fragments merge to one range, out-of-order fragments still cover the timeline, a lone fragment yields its own half-range.
Media	Honest scope: this lands the container-parsing half of MP4 streaming. isTypeSupported still returns false for video/mp4 codecs, because Floati has no H.264/AV1/AAC decoder yet (those are patent-encumbered or need a heavy dependency) - so no page is told MP4 will play. The demuxer is the foundation a decoder plugs into later. Whole conformance suite unchanged. Research dossier docs/next-floors-research.md.