added default spawn-location

doc/RFC_XR_Fragments.md

@@ -146,6 +146,7 @@ XR Fragments allows us to better use implicit metadata inside 3D scene(files), b
 1. [Accessibility interface](#accessibility-interface)
   1. [Two-button navigation](#two-button-navigation)
   1. [Overlap with fileformat-specific extensions](#overlap-with-fileformat-specific-extensions)
+1. [XRF microformat](#XRF-microformat)
 1. [Vendor Prefixes](#vendor-prefixes)
 1. [Security Considerations](#security-considerations)
 1. [FAQ](#faq)
@@ -869,6 +870,111 @@ Therefore a 2-button navigation-interface is the bare minimum interface:
 2. objects with href metadata can be activated via a key (enter on a keyboard)
 3. the TTS reads the href-value (and/or aria-description if available)
 
+## XRF microformat
+
+How can applications discover 3D experiences on a network?
+
+> Answer: **spatial microformats** 
+
+The XRF microformat is an [optional](#Progressive%20enhancement) text heuristics which applications can detect across various usecases.
+
+#### via HTML webpage
+
+If the browser/application requests an webpage (`https://nlnet.nl` e.g.) it should check for the [rel-me microformat](https://gmpg.org/xfn/) :
+
+```
+<link rel="alternate" as="spatial-entrypoint" href="scene.xrf.glb">
+```
+
+This way the application loads `https://nlnet.nl/scene.xrf.glb` when the user types `nlnet.nl` into the URLbar.<br>
+Optionally, `type` can be specified for dynamically generated 3D files:
+
+```
+<link rel="alternate" as="spatial-entrypoint" href="https://worlds.org/scene.php#platformB" type="model/gltf+binary" />
+```
+
+The `type`-attribute is for fallback-purposes.<br>
+Viewer-supported 3D file-extensions (`.glb` e.g.) will **ALWAYS** take precedence over the (non)presence of the `type` attribute.<br>
+The reason is that platforms (Mastodon 'labels' e.g.) don't allow specifying type-attributes.<br>
+Another reason is that XR Fragments is filetype-agnostic, so flexibility is expected on the viewer-side.
+
+> NOTE: in case of multiple 3D files mentioned in `<link rel="me"`, only the first (supported 3D filetype) will be chosen.
+
+Example of multiple spatial microformats:
+
+```
+<link rel="alternate" as="spatial-entrypoint" href="scene.xrf.glb"/>
+<link rel="me" href="myavatar.vrm"/>
+<!-- JanusXR microformat https://github.com/jbaicoianu/janusweb
+   <FireBoxRoom>
+      <Assets>
+        <assetobject id="experience" src="scene.xrf.glb"/>
+      </Assets>
+      <Room>
+        <object pos="0 0 0" collision_id="experience" id="experience" />
+      </Room>
+   </FireBoxRoom>
+-->
+```
+
+#### via WebFinger
+
+When John has an account on foo.com, how can other applications request his 3D homepage by simply entering `john@foo.com`?
+
+> Answer: it can be requested at `https://foo.com/.well-known/webfinger?resource=acct:john@foo.com`, resulting in:
+
+```
+{
+  "subject": "acct:john@foo.com",
+  "aliases": [
+    "https://mastodon.example/social/john",
+    "https://john.foo.com",
+    "https://3d.john.foo.com/model/scene.glb"
+  ],
+  "properties": {
+    "http://schema.org/name": "John Doe",
+    "http://schema.org/description": "Developer, 3D Enthusiast, and Social Explorer"
+  },
+  "links": [
+    {
+      "rel": "http://ostatus.org/schema/1.0/subscribe",
+      "template": "https://mastodon.example/social/john/{uri}"
+    },
+    {
+      "rel": "self",
+      "type": "text/html",
+      "href": "https://john.foo.com"
+    },
+    {
+      "rel": "me",
+      "type": "text/html",
+      "href": "https://john.foo.com"
+    },
+		{
+      "rel": "me",
+      "type": "model/gltf+binary",
+      "href": "https://3d.john.foo.com/model/avatar.vrm"
+    },
+    {
+      "rel": "scene",
+      "type": "model/gltf+binary",
+      "href": "https://3d.john.foo.com/model/scene.xrf.glb"
+    }
+  ]
+}
+```
+
+This way the application will load `https://3d.john.foo.com/model/scene.glb` when the user types `john@foo.com` into the user field.
+
+#### via Text (URI)
+
+Another way for an application to trigger loading a 3D scene is by detecting URI's of 3D scene-files any text:
+
+* `foo.glb` (or any other popular 3D extension)
+* `https://foo.com/scene.glb` (or any other popular protocol)
+
+This way, the application can highlight the link whenever it detects the URI (in a text-file or text-section of a 3D model)
+
 ## Overlap with fileformat-specific extensions 
 
 Some 3D scene-fileformats have support for extensions.