Merge branch 'main' into mimeSniff

.gitattributes

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+*.bs linguist-language=HTML

.github/workflows/auto-publish.yml

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+# Workflow based on the main w3c/spec-prod action example:
+# https://github.com/w3c/spec-prod/#basic-usage
+
+name: Build, Validate, Deploy and Publish
+
+on:
+  # Worflow runs on pull requests where it makes sure that the spec can still be
+  # generated, that markup is valid and that there are no broken links, as
+  # well as on pushes to the default branch where it also deploys the generated
+  # spec to the gh-pages branch and publishes the result to /TR.
+  # The "workflow_dispatch" hook allows admins to also trigger the workflow
+  # manually from GitHub's UI.
+  pull_request: {}
+  push:
+    branches: [main]
+  workflow_dispatch:
+
+jobs:
+  main:
+    runs-on: ubuntu-20.04
+    steps:
+      # See doc at https://github.com/actions/checkout#checkout-v2
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      # See doc at https://github.com/w3c/spec-prod/#spec-prod
+      # The action only deploys the generated spec to the gh-pages branch when
+      # the workflow was triggered by a push to the default branch.
+      - name: Build and validate index.html, push to gh-pages branch if needed
+        uses: w3c/spec-prod@v2
+        with:
+          GH_PAGES_BRANCH: gh-pages
+          W3C_ECHIDNA_TOKEN: ${{ secrets.ECHIDNA_TOKEN }}
+          W3C_WG_DECISION_URL: https://github.com/w3c/media-wg/issues/27
+          W3C_BUILD_OVERRIDE: |
+            status: WD

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 Contributions to this repository are intended to become part of Recommendation-track documents
 governed by the [W3C Patent Policy](http://www.w3.org/Consortium/Patent-Policy-20040205/) and
-[Document License](http://www.w3.org/Consortium/Legal/copyright-documents). To contribute, you must
+[Software and Document License](http://www.w3.org/Consortium/Legal/copyright-software). To contribute, you must
 either participate in the relevant W3C Working Group or make a non-member patent licensing
  commitment.
 

LICENSE.md

@@ -1,2 +1,2 @@
-All documents in this Repository are licensed by contributors under the [W3C Document
-License](http://www.w3.org/Consortium/Legal/copyright-documents).
+All documents in this Repository are licensed by contributors under the [W3C
+Software and Document License](http://www.w3.org/Consortium/Legal/copyright-software).

hdr_explainer.md

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+# HDR Capability Detection
+HDR capabilities are detected with a combination of CSS and MediaCapabilities APIs.  Both APIs should be called to determine if HDR can be correctly presented. 
+
+Please note that both of these APIs are fairly new and are not yet implemented by all user agents. See Implementations section below.
+
+# CSS: is the display HDR capable?
+
+Media Queries Level 5 defines a `dynamic-range` feature, with values `"standard"`  and `"high"`. 
+
+https://www.w3.org/TR/mediaqueries-5/#descdef-media-dynamic-range
+
+```
+let isScreenHDR = window.matchMedia('(dynamic-range: high)').matches;
+```
+
+This indicates whether the current display is HDR capable. 
+
+# CSS: video-* prefixed MediaQueries
+User agents on TVs tend to offer video capabilities at the native resolution and dynamic range (ex: 4k HDR) while offering much lower capabilities for the rest of the web page (maybe 1080p SDR). To surface these distinct capabilities, Media Queries Level 5 defines features prefixed by "video-".
+
+https://www.w3.org/TR/mediaqueries-5/#video-prefixed-features
+
+This includes `video-dynamic-range`. On televisions, this value may be `"high"` while the non-prefixed `dynamic-range` remains `"standard"`. 
+
+On desktop and mobile devices, values for video-* features will generally match their non-prefixed counterparts. 
+
+The sizing video-\* prefixed features are still under discussion, including `video-width`, `video-height`, and `video-resolution`. See https://github.com/w3c/csswg-drafts/issues/5044
+
+
+# MediaCapabilities: does the UA support decoding/rendering my brand of HDR video?
+Use MediaCapabilities to ask about HDR video decoding/rendering capabilities, **independent of the display capabilities**.
+* Does it support the desired transfer function? https://www.w3.org/TR/media-capabilities/#transferfunction
+* Does it support rendering in the desired color gamut? https://www.w3.org/TR/media-capabilities/#colorgamut
+* Does it support the desired HDR metadata? https://www.w3.org/TR/media-capabilities/#hdrmetadatatype
+
+Here's a code sample (note the last 3 fields in `hdrConfig`).
+```
+let hdrConfig = {
+  type: 'media-source',
+  video: {
+    contentType: 'video/webm; codecs="vp09.00.10.08"',
+    width: 1920,
+    height: 1080,
+    bitrate: 2646242,
+    framerate: '25',
+    transferFunction: 'pq',
+    colorGamut: 'p3',
+    hdrMetadataType: 'smpteSt2086',
+  }
+};
+
+navigator.mediaCapabilities.decodingInfo(hdrConfig).then(function(info) {
+	if (info.supported) {
+		// Playback is supported! See also, info.smooth and info.powerEfficient. 
+	}
+});
+```
+
+# Implementations
+
+As of 2020/11, here's the unofficial status of user agent implementations for the above:
+* CSS dynamic-range is implemented by Safari. Not yet started by Chrome (ETA early 2021), nor Firefox
+* CSS video-* features are not yet implemented anywhere.
+* MediaCapabilities HDR inputs are implemented by Safari. Chrome's implementation is work-in-progress. Not yet started in Firefox.