diff --git a/docs/custom-media.md b/docs/custom-media.md
new file mode 100644
index 00000000..beee96b1
--- /dev/null
+++ b/docs/custom-media.md
@@ -0,0 +1,160 @@
+CyTube Custom Content Metadata
+==============================
+
+Last Updated: 2017-08-08
+
+Status: Experimental
+
+## Purpose ##
+
+CyTube currently supports adding custom audio/video content by allowing the user
+to supply a direct URL to an audio/video file.  The server uses `ffprobe` to
+probe the file for various metadata, including the codec/container format and
+the duration.  This approach has a few disadvantages over the officially
+supported media providers, namely:
+
+  * Since it accepts a single file, it is not possible to provide multiple
+    source URLs with varying formats or bitrates to allow viewers to select the
+    best source for their computer.
+    - It also means it is not possible to provide text tracks for subtitles or
+      closed captioning, or to provide image URLs for thumbnails/previews.
+  * Probing the file with `ffprobe` is slow, especially if the content is hosted
+    in a far away network location, which at best is inconvenient and at worst
+    results in timeouts and inability to add the content.
+  * Parsing the `ffprobe` output is inexact, and may sometimes result in
+    detecting the wrong format, or failing to detect the title.
+
+This document specifies a new supported media provider which allows users to
+provide a JSON manifest specifying the metadata for custom content in a way that
+avoids the above issues and is more flexible for extension.
+
+## Manifest Format ##
+
+To add custom content, the user provides a JSON object with the following keys:
+
+  * `title`: A nonempty string specifying the title of the content.  For legacy
+    reasons, CyTube currently truncates this to 100 UTF-8 characters.
+  * `duration`: A non-negative, finite number specifying the duration, in
+    seconds, of the content.  This is what the server will use for timing
+    purposes.  Decimals are allowed, but CyTube's timer truncates the value as
+    an integer number of seconds, so including fractional seconds lends no
+    advantage.
+  * `live`: An optional boolean (default: `false`) indicating whether the
+    content is live or pre-recorded.  For live content, the `duration` is
+    ignored, and the server won't advance the playlist automatically.
+  * `thumbnail`: An optional string specifying a URL for a thumbnail image of
+    the content.  CyTube currently does not support displaying thumbnails in the
+    playlist, but this functionality may be offered in the future.
+  * `sources`: A nonempty list of playable sources for the content.  The format
+    is described below.
+  * `textTracks`: An optional list of text tracks for subtitles or closed
+    captioning.  The format is described below.
+
+### Source Format ###
+
+Each source entry is a JSON object with the following keys:
+
+  * `url`: A valid URL that browsers can use to retrieve the content.  The URL
+    must resolve to a publicly-routed IP address, and must the `https:` scheme.
+  * `contentType`: A string representing the MIME type of the content at `url`.
+    A list of acceptable MIME types is provided below.
+  * `quality`: A number representing the quality level of the source.  The
+    supported quality levels are `240`, `360`, `480`, `540`, `720`, `1080`,
+    `1440`, and `2160`.  This may be extended in the future.
+  * `bitrate`: An optional number indicating the bitrate (in Kbps) of the
+    content.  It must be a positive, finite number if provided.  The bitrate is
+    not currently used by CyTube, but may be used by extensions or custom
+    scripts to determine whether this source is feasible to play on the viewer's
+    internet connection.
+
+#### Acceptable MIME Types ####
+
+The following MIME types are accepted for the `contentType` field:
+
+  * `video/mp4`
+  * `video/webm`
+  * `video/ogg`
+  * `application/x-mpegURL` (HLS streams)
+    - HLS is only supported for livestreams.  Metadata with HLS sources
+      but without `live: true` will be rejected.
+  * ~~`rtmp/flv`~~
+    - In light of Adobe phasing out support for Flash, and many browsers
+      already dropping support, RTMP is not supported by this feature.
+      RTMP streams are only supported through the existing `rt:` media
+      type.
+  * `audio/aac`
+  * `audio/ogg`
+  * `audio/mpeg`
+
+Other audio or video formats, such as AVI, MKV, and FLAC, are not supported due
+to lack of common support across browsers for playing these formats.  For more
+information, refer to
+[MDN](https://developer.mozilla.org/en-US/docs/Web/HTML/Supported_media_formats#Browser_compatibility).
+
+### Text Track Format ###
+
+Each text track entry is a JSON object with the following keys:
+
+
+  * `url`: A valid URL that browsers can use to retrieve the track.  The URL
+    must resolve to a publicly-routed IP address, and must the `https:` scheme.
+  * `contentType`: A string representing the MIME type of the track at `url`.
+    The only currently supported MIME type is
+    [`text/vtt`](https://developer.mozilla.org/en-US/docs/Web/API/WebVTT_API).
+  * `name`: A name for the text track.  This is displayed in the menu for the
+    viewer to select a text track.
+
+**Important note regarding text tracks and CORS:**
+
+By default, browsers block requests for WebVTT tracks hosted on different
+domains than the current page.  In order for text tracks to work cross-origin,
+the `Access-Control-Allow-Origin` header needs to be set by the remote server
+when serving the VTT file.  See
+[MDN](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Origin)
+for more information about setting this header.
+
+## Example ##
+
+    {
+      "title": "Test Video",
+      "duration": 10,
+      "live": false,
+      "thumbnail": "https://example.com/thumb.jpg",
+      "sources": [
+        {
+          "url": "https://example.com/video.mp4",
+          "contentType": "video/mp4",
+          "quality": 1080,
+          "bitrate": 5000
+        }
+      ],
+      "textTracks": [
+        {
+          "url": "https://example.com/subtitles.vtt",
+          "contentType": "text/vtt",
+          "name": "English Subtitles"
+        }
+      ]
+    }
+
+## Permissions ##
+
+The permission node to allow users to add custom content is the same as the
+permission node for the existing raw file support.  Custom content is considered
+as an extension of the existing feature.
+
+## Unsupported/Undefined Behavior ##
+
+The behavior under any the following circumstances is not defined by this
+specification, and any technical support in these cases is voided.  This list is
+non-exhaustive.
+
+  * Source URLs or text track URLs are hosted on a third-party website that does
+    not have knowledge of its content being played on CyTube.
+  * The webserver hosting the source or text track URLs serves a different MIME
+    type than the one specified in the manifest.
+  * The webserver hosting the source or text track URLs serves a file that does
+    not match the MIME type specified in the `Content-Type` HTTP header returned
+    to the browser.
+  * The manifest includes source URLs or text track URLs with expiration times,
+    session IDs, etc. in the URL querystring.
diff --git a/src/custom-media.js b/src/custom-media.js
index 149f6b0d..68482e07 100644
--- a/src/custom-media.js
+++ b/src/custom-media.js
@@ -29,6 +29,10 @@ const SOURCE_CONTENT_TYPES = new Set([
     'video/webm'
 ]);
 
+const LIVE_ONLY_CONTENT_TYPES = new Set([
+    'application/x-mpegURL'
+]);
+
 export function lookup(url, opts) {
     if (!opts) opts = {};
     if (!opts.hasOwnProperty('timeout')) opts.timeout = 10000;
@@ -158,11 +162,11 @@ export function validate(data) {
         validateURL(data.thumbnail);
     }
 
-    validateSources(data.sources);
+    validateSources(data.sources, data);
     validateTextTracks(data.textTracks);
 }
 
-function validateSources(sources) {
+function validateSources(sources, data) {
     if (!Array.isArray(sources))
         throw new ValidationError('sources must be a list');
     if (sources.length === 0)
@@ -178,6 +182,11 @@ function validateSources(sources) {
                 `unacceptable source contentType "${source.contentType}"`
             );
 
+        if (LIVE_ONLY_CONTENT_TYPES.has(source.contentType) && !data.live)
+            throw new ValidationError(
+                `contentType "${source.contentType}" requires live: true`
+            );
+
         if (!SOURCE_QUALITIES.has(source.quality))
             throw new ValidationError(`unacceptable source quality "${source.quality}"`);
 
diff --git a/test/custom-media.js b/test/custom-media.js
index 30d8d4e9..96922432 100644
--- a/test/custom-media.js
+++ b/test/custom-media.js
@@ -89,6 +89,16 @@ describe('custom-media', () => {
 
             assert.throws(() => validate(invalid), /URL protocol must be HTTPS/);
         });
+
+        it('rejects non-live HLS', () => {
+            invalid.live = false;
+            invalid.sources[0].contentType = 'application/x-mpegURL';
+
+            assert.throws(
+                () => validate(invalid),
+                /contentType "application\/x-mpegURL" requires live: true/
+            );
+        });
     });
 
     describe('#validateSources', () => {