diff --git a/specification/index.bs b/specification/index.bs
index 9b682f0a1..47fb6b875 100644
--- a/specification/index.bs
+++ b/specification/index.bs
@@ -9,6 +9,7 @@ Editor: Mukul Purohit, Microsoft Corporation https://www.microsoft.com, mpurohit
 Editor: Tomislav Jovanovic, Mozilla https://www.mozilla.org/, tjovanovic@mozilla.com
 Editor: Oliver Dunk, Google https://www.google.com, oliverdunk@chromium.org
 Abstract: [Placeholder] Abstract.
+Repository: w3c/webextensions
 Markup Shorthands: markdown yes
 </pre>
 
@@ -108,6 +109,8 @@ This key must be present if the `_locales` subdirectory is present, must be abse
 
 This key may be present.
 
+Issue: Specify background scripts. For relevant discussion, see https://github.com/w3c/webextensions/issues/282
+
 ### Key `commands`
 
 This key may be present.
@@ -158,15 +161,64 @@ The <a href="#key-trial_tokens">`trial_tokens`</a> key is an optional [=list=] o
 
 Filenames beginning with an underscore (`_`) are reserved for use by user agent.
 
-# Isolated worlds
+# Execution contexts
+
+Extensions can execute JavaScript code, in any of the following execution contexts:
+
+* An <dfn>extension context</dfn> is a [=realm=] associated with an [=extension origin=].
+* A <dfn>privileged extension context</dfn> is an [=extension context=] with access to the full set of extension APIs available to the extension. For example, the background page or worker defined through [[#key-background]].
+* A <dfn>content script context</dfn> is an [=extension context=] and [=isolated world=] with limited access to a subset of extension APIs. This is the default execution environment of all [=content scripts=] of an extension.
+* The [=main world=] of a web page not associated with an [=extension origin=] is not an extension context. It does not have access to any extension API, except when an extension allows so through [[#key-externally_connectable]].
+
+Some extension APIs may involve the execution of JavaScript code in contexts other than what is specified above. For example, the `userScripts` API allows the creation of `USER_SCRIPT` worlds that are isolated similarly to [=isolated worlds=] but with distinct API availability.
+
+## Isolated worlds
+
+A <dfn>world</dfn> is a [=realm=] with its own [=global object=].
+
+The <dfn>main world</dfn> is the [=realm=] whose [=global object=] is the
+associated document's {{Window}}, in which the document's own scripts run.
+This is the realm implied throughout other specifications that assume a single
+realm per document.
+
+A document may also have a number of [=isolated worlds=], created by the user
+agent to run [=content scripts=] in a [=content script context=].
+
+An <dfn>isolated world</dfn> is a distinct [=realm=] whose [=global object=]'s
+interface is {{Window}}, associated with the [=main world=]'s document. The
+[=platform objects=] of this realm are distinct from their counterparts in the
+[=main world=], but operate on the same underlying state. These operations
+should maintain isolation across realms: no object in an [=isolated world=]'s
+realm is observable from the [=main world=].
+
+<div class="note">
+For example, {{CustomEvent}} specifies the `event.detail` attribute that
+"must return the value it was initialized to". Event dispatch can cross worlds,
+and following this requirement to the letter would result in the exposure of
+an object from one realm to another. Potential resolutions include:
+
+- Throwing at `CustomEvent` construction when a non-primitive value is passed.
+- Returning `null`.
+- Returning a structured clone, per [[HTML#safe-passing-of-structured-data]].
+- Returning a redacted version of the object.
 
-<dfn>Worlds</dfn> are isolated JavaScript contexts with access to the same underlying DOM tree but their own set of wrappers around those DOM objects. Declarations in the global scope are also isolated.
+</div>
 
 # Unavailable APIs
 
 # The `browser` global
 
-# Extension origin
+[[webextensions-browser-global inline]] is the primary namespace hosting extension APIs, available to [=extension contexts=].
+
+Although the [=main world=] of a web page is not an [=extension context=], it may also contain the `browser` global to offer access to functionality granted by [[#key-externally_connectable]].
+
+# <dfn>Extension origin</dfn>
+
+The extension origin is a [=tuple origin=] consisting of an [=extension scheme=] and an extension-specific host. An <dfn>extension scheme</dfn> is a browser-specific scheme reserved for extension use.
+
+<div class="example">
+Examples of [=extension schemes=] include `chrome-extension`, `moz-extension`, and `safari-web-extension`.
+</div>
 
 # Localization
 
@@ -202,9 +254,9 @@ A <dfn>glob</dfn> can be any [=string=]. It can contain any number of wildcards
 
 ## Background content
 
-## Content scripts
+## <dfn>Content scripts</dfn>
 
-<dfn>Content scripts</dfn> represent a set of JS and CSS files that should be injected into matching pages loaded by the user agent. They are injected using the steps in [[#inject-a-content-script]].
+Content scripts represent a set of JS and CSS files that should be injected into matching pages loaded by the user agent. They are injected using the steps in [[#inject-a-content-script]]. Content scripts run in an [=isolated world=] by default.
 
 ### Key `matches`
 
@@ -277,6 +329,10 @@ enum ExecutionWorld {
 
 The {{ExecutionWorld}} enum represents a JavaScript [=world=].
 
+"ISOLATED" corresponds to an [=isolated world=].
+
+"MAIN" corresponds to the [=main world=].
+
 ## Extension pages
 
 # Classes of security risk
@@ -336,3 +392,24 @@ To determine if a content script should be injected in a document:
 1. If |url| matches an entry in `exclude_matches` or `exclude_globs`, return.
 1. If this is a child frame, and `all_frames` is not `true`, return.
 1. Otherwise, inject the content script. This should be done based on the `run_at` setting.
+
+<pre class="link-defaults">
+spec:html; type:dfn; for:realm; text:global object
+</pre>
+
+<pre class="biblio">
+{
+  "webextensions-browser-global": {
+    "authors": [
+      "Patrick Kettner"
+    ],
+    "href": "https://w3c.github.io/webextensions/specification/window.browser.html",
+    "title": "window.browser",
+    "status": "CG-DRAFT",
+    "publisher": "WECG",
+    "deliveredBy": [
+      "https://www.w3.org/groups/cg/webextensions/"
+    ]
+  }
+}
+</pre>
diff --git a/specification/window.browser.bs b/specification/window.browser.bs
index e46105343..52621cb0a 100644
--- a/specification/window.browser.bs
+++ b/specification/window.browser.bs
@@ -1,6 +1,6 @@
 <pre class="metadata">
 Title: window.browser
-Shortname: wecg-browser
+Shortname: webextensions-browser-global
 Level: 1
 Group: wecg
 Status: w3c/CG-DRAFT