docs: add supported features doc

docs/supported-features.md

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+# Supported Features
+
+## Browsers
+
+Any browser that supports [MSE] (media source extensions). See
+https://caniuse.com/#feat=mediasource
+
+Note that browsers with native HLS support may play content with the native player, unless
+the [overrideNative] option is used. Some notable browsers with native HLS players are:
+
+* Safari (macOS and iOS)
+* Chrome Android
+* Firefox Android
+
+However, due to the limited features offered by some of the native players, the only
+browser on which VHS defaults to using the native player is Safari (macOS and iOS).
+
+## Streaming Formats and Media Types
+
+### Streaming Formats
+
+VHS aims to be mostly streaming format agnostic. So long as the manifest can be parsed to
+a common JSON representation, VHS should be able to play it. However, due to some large
+differences between the major streaming formats (HLS and DASH), some format specific code
+is included in VHS. If you have another format you would like supported, please reach out
+to us (e.g., file an issue).
+
+* [HLS] (HTTP Live Streaming)
+* [MPEG-DASH] (Dynamic Adaptive Streaming over HTTP)
+
+### Media Container Formats
+
+* [TS] (MPEG Transport Stream)
+* [MP4] (MPEG-4 Part 14: MP4, M4A, M4V, M4S, MPA), ISOBMFF
+* [AAC] (Advanced Audio Coding)
+
+### Codecs
+
+If the content is packaged in an [MP4] container, then any codec supported by the browser
+is supported. If the content is packaged in a [TS] container, then the codec must be
+supported by [the transmuxer]. The following codecs are supported by the transmuxer:
+
+* [AVC] (Advanced Video Coding, h.264)
+* [AVC1] (Advnced Video Coding, h.265)
+* [HE-AAC] (High Efficiency Advanced Audio Coding, mp4a.40.5)
+* LC-AAC (Low Complexity Advanced Audio Coding, mp4a.40.2)
+
+## General Notable Features
+
+The following is a list of some, but not all, common streaming features supported by VHS.
+It is meant to highlight some common use cases (and provide for easy searching), but is
+not meant serve as an exhaustive list.
+
+* VOD (video on demand)
+* LIVE
+* Multiple audio tracks
+* Timed [ID3] Metadata is automatically translated into HTML5 metedata text tracks
+* Cross-domain credentials support with [CORS]
+* Any browser supported resolution (e.g., 4k)
+* Any browser supported framerate (e.g., 60fps)
+* [DRM] via [videojs-contrib-eme]
+* Audio only
+* Video only
+* In-manifest [WebVTT] subtitles are automatically translated into standard HTML5 subtitle
+  tracks
+* [AES-128] and SAMPLE-AES segment encryption
+
+## Notable Missing Features
+
+Note that the following features have not yet been implemented or may work but are not
+currently suppported in browsers that do not rely on the native player. For browsers that
+use the native player (e.g., Safari for HLS), please refer to their documentation.
+
+### Container Formats
+
+* [WebM]
+* [WAV]
+* [MP3]
+* [OGG]
+
+### Codecs
+
+If the content is packaged within an [MP4] container and the browser supports the codec, it
+will play. However, the following are some codecs that are not routinely tested, or are not
+supported when packaged within [TS].
+
+* [MP3]
+* [Vorbis]
+* [WAV]
+* [FLAC]
+* [Opus]
+* [VP8]
+* [VP9]
+* [Dolby Vision] (DVHE)
+* [Dolby Digital] Audio (AC-3)
+* [Dolby Digital Plus] (E-AC-3)
+
+### HLS Missing Features
+
+Note: features for low latency HLS in the [2nd edition of HTTP Live Streaming] are on the
+roadmap, but not currently available.
+
+VHS strives to support all of the features in the HLS specification, however, some have
+not yet been implemented. VHS currently supports everything in the
+[HLS specification v7, revision 23], except the following:
+
+* Use of [EXT-X-MAP] with [TS] segments
+  * [EXT-X-MAP] is currently supported for [MP4] segments, but not yet for TS
+* I-Frame playlists via [EXT-X-I-FRAMES-ONLY] and [EXT-X-I-FRAME-STREAM-INF]
+* [MP3] Audio
+* [Dolby Digital] Audio (AC-3)
+* [Dolby Digital Plus] Audio (E-AC-3)
+* KEYFORMATVERSIONS of [EXT-X-KEY]
+* [EXT-X-DATERANGE]
+* [EXT-X-SESSION-DATA]
+* [EXT-X-SESSION-KEY]
+* [EXT-X-INDEPENDENT-SEGMENTS]
+* Use of [EXT-X-START] (value parsed but not used)
+* Alternate video via [EXT-X-MEDIA] of type video
+* ASSOC-LANGUAGE in [EXT-X-MEDIA]
+* CHANNELS in [EXT-X-MEDIA]
+* Use of AVERAGE-BANDWIDTH in [EXT-X-STREAM-INF] (value parsed but not used)
+* Use of FRAME-RATE in [EXT-X-STREAM-INF] (value parsed but not used)
+* Use of HDCP-LEVEL in [EXT-X-STREAM-INF]
+
+In the event of encoding changes within a playlist (see
+https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-6.3.3), the
+behavior will depend on the browser.
+
+### DASH Missing Features
+
+DASH support is more recent than HLS support in VHS, however, VHS strives to achieve as
+complete compatibility as possible with the DASH spec. The following are some notable
+features in the DASH specification that are not yet implemented in VHS:
+
+Note that many of the following are parsed by [mpd-parser] but are either not yet used, or
+simply take on their default values (in the case where they have valid defaults).
+
+* Audio rendition switching
+  * Each video rendition is paired with an audio rendition for the duration of playback.
+* MPD
+  * @id
+  * @profiles
+  * @availabilityStartTime
+  * @availabilityEndTime
+  * @minBufferTime
+  * @maxSegmentDuration
+  * @maxSubsegmentDuration
+  * ProgramInformation
+  * Location
+  * Metrics
+* Period
+  * @xlink:href
+  * @xlink:actuate
+  * @id
+  * @duration
+    * Normally used for determing the PeriodStart of the next period, VHS instead relies
+      on segment durations to determine timing of each segment and timeline
+  * @bitstreamSwitching
+  * Subset
+* AdaptationSet
+  * @xlink:href
+  * @xlink:actuate
+  * @id
+  * @group
+  * @par (picture aspect ratio)
+  * @minBandwidth
+  * @maxBandwidth
+  * @minWidth
+  * @maxWidth
+  * @minHeight
+  * @maxHeight
+  * @minFrameRate
+  * @maxFrameRate
+  * @segmentAlignment
+  * @bitstreamSwitching
+  * @subsegmentAlignment
+  * @subsegmentStartsWithSAP
+  * Accessibility
+  * Rating
+  * Viewpoint
+  * ContentComponent
+* Representation
+  * @id (used for SegmentTemplate but not exposed otherwise)
+  * @qualityRanking
+  * @dependencyId (dependent representation)
+  * @mediaStreamStructureId
+  * SubRepresentation
+* CommonAttributesElements (for AdaptationSet, Representation and SubRepresentation elements)
+  * @profiles
+  * @sar
+  * @frameRate
+  * @audioSamplingRate
+  * @segmentProfiles
+  * @maximumSAPPeriod
+  * @startWithSAP
+  * @maxPlayoutRate
+  * @codingDependency
+  * @scanType
+  * FramePacking
+  * AudioChannelConfiguration
+* SegmentBase
+  * @presentationTimeOffset
+  * @indexRangeExact
+  * RepresentationIndex
+* MultipleSegmentBaseInformation elements
+* SegmentList
+  * @xlink:href
+  * @xlink:actuate
+  * MultipleSegmentBaseInformation
+  * SegmentURL
+    * @index
+    * @indexRange
+* SegmentTemplate
+  * MultipleSegmentBaseInformation
+  * @index
+  * @bitstreamSwitching
+* BaseURL
+  * @serviceLocation
+* Template-based Segment URL construction
+  * Live DASH assets that use $Time$ in a SegmentTemplate, and also have a SegmentTimeline
+    where only the first S has a t and the rest only have a d do not update on playlist
+    refreshes
+    See: https://github.com/videojs/http-streaming#dash-assets-with-time-interpolation-and-segmenttimelines-with-no-t
+* ContentComponent elements
+  * Right now manifests are assumed to have a single content component, with the properties
+    described directly on the AdaptationSet element
+* SubRepresentation elements
+* Subset elements
+* Early Available Periods (may work, but has not been tested)
+* Access to subsegments via a subsegment index ('ssix')
+* The @profiles attribute is ignored (best support for all profiles is attempted, without
+  consideration of the specific profile). For descriptions on profiles, see section 8 of
+  the DASH spec.
+* Construction of byte range URLs via a BaseURL byteRange template (Annex E.2)
+* Multiperiod content where the representation sets are not the same across periods
+* In the event that an S element has a t attribute that is greater than what is expected,
+  it is not treated as a discontinuity, but instead retains its segment value, and may
+  result in a gap in the content
+
+[MSE]: https://www.w3.org/TR/media-source/
+[HLS]: https://en.wikipedia.org/wiki/HTTP_Live_Streaming
+[MPEG-DASH]: https://en.wikipedia.org/wiki/Dynamic_Adaptive_Streaming_over_HTTP
+[TS]: https://en.wikipedia.org/wiki/MPEG_transport_stream
+[MP4]: https://en.wikipedia.org/wiki/MPEG-4_Part_14
+[AAC]: https://en.wikipedia.org/wiki/Advanced_Audio_Coding
+[AVC]: https://en.wikipedia.org/wiki/Advanced_Video_Coding
+[AVC1]: https://en.wikipedia.org/wiki/Advanced_Video_Coding
+[HE-AAC]: https://en.wikipedia.org/wiki/High-Efficiency_Advanced_Audio_Coding
+[ID3]: https://en.wikipedia.org/wiki/ID3
+[CORS]: https://en.wikipedia.org/wiki/Cross-origin_resource_sharing
+[DRM]: https://en.wikipedia.org/wiki/Digital_rights_management
+[WebVTT]: https://www.w3.org/TR/webvtt1/
+[AES-128]: https://en.wikipedia.org/wiki/Advanced_Encryption_Standard
+[WebM]: https://en.wikipedia.org/wiki/WebM
+[WAV]: https://en.wikipedia.org/wiki/WAV
+[MP3]: https://en.wikipedia.org/wiki/MP3
+[OGG]: https://en.wikipedia.org/wiki/Ogg
+[Vorbis]: https://en.wikipedia.org/wiki/Vorbis
+[FLAC]: https://en.wikipedia.org/wiki/FLAC
+[Opus]: https://en.wikipedia.org/wiki/Opus_(audio_format)
+[VP8]: https://en.wikipedia.org/wiki/VP8
+[VP9]: https://en.wikipedia.org/wiki/VP9
+
+[overrideNative]: https://github.com/videojs/http-streaming#overridenative
+[the transmuxer]: https://github.com/videojs/mux.js
+[videojs-contrib-eme]: https://github.com/videojs/videojs-contrib-eme
+
+[2nd edition of HTTP Live Streaming]: https://tools.ietf.org/html/draft-pantos-hls-rfc8216bis-07.html
+[HLS specification v7, revision 23]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23
+
+[EXT-X-MAP]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.2.5
+[EXT-X-STREAM-INF]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.4.2
+[EXT-X-SESSION-DATA]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.4.4
+[EXT-X-DATERANGE]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.2.7
+[EXT-X-KEY]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.2.7
+[EXT-X-I-FRAMES-ONLY]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.3.6
+[EXT-X-I-FRAME-STREAM-INF]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.4.3
+[EXT-X-SESSION-KEY]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.4.5
+[EXT-X-INDEPENDENT-SEGMENTS]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.5.1
+[EXT-X-START]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.5.2
+[EXT-X-MEDIA]: https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.4.1
+
+[Dolby Vision]: https://en.wikipedia.org/wiki/High-dynamic-range_video#Dolby_Vision
+[Dolby Digital]: https://en.wikipedia.org/wiki/Dolby_Digital
+[Dolby Digital Plus]: https://en.wikipedia.org/wiki/Dolby_Digital_Plus
+
+[mpd-parser]: https://github.com/videojs/mpd-parser