Re: [heycam/webidl] Define the `[CrossOriginIsolated]` extended attribute. (#883)

@annevk approved this pull request.

Thanks this looks good. I rather not call things PowerfulFeature as that tends to create confusion.

Are the 4 newlines before a `<h4>` intended?

> +Note: This is because adding the [{{CrossOriginIsolated}}] [=extended attribute=] on a [=member=]
+when its containing definition is also annotated with the [{{CrossOriginIsolated}}]
+[=extended attribute=] does not further restrict the exposure of the [=member=].
+
+An [=interface=] without the [{{CrossOriginIsolated}}] [=extended attribute=]
+must not [=interface/inherit=] from another interface
+that does specify [{{CrossOriginIsolated}}].
+
+<div class="example">
+
+    The following [=IDL fragment=] defines an interface with one [=operation=] that is executable
+    from all contexts, and two which are executable only from cross-origin isolated contexts.
+
+    <pre highlight="webidl">
+        [Exposed=Window]
+        interface PowerfulFeature {

```suggestion
        interface ExampleFeature {
```

> +must not [=interface/inherit=] from another interface
+that does specify [{{CrossOriginIsolated}}].
+
+<div class="example">
+
+    The following [=IDL fragment=] defines an interface with one [=operation=] that is executable
+    from all contexts, and two which are executable only from cross-origin isolated contexts.
+
+    <pre highlight="webidl">
+        [Exposed=Window]
+        interface PowerfulFeature {
+          // This call will succeed in all contexts.
+          Promise &lt;Result&gt; calculateNotSoSecretResult();
+
+          // This operation will not be exposed to a non-isolated context. In such a context,
+          // there will be no "calculateSecretResult" property on PowerfulFeature.prototype.

```suggestion
          // there will be no "calculateSecretResult" property on ExampleFeature.prototype.
```

> +    The following [=IDL fragment=] defines an interface with one [=operation=] that is executable
+    from all contexts, and two which are executable only from cross-origin isolated contexts.
+
+    <pre highlight="webidl">
+        [Exposed=Window]
+        interface PowerfulFeature {
+          // This call will succeed in all contexts.
+          Promise &lt;Result&gt; calculateNotSoSecretResult();
+
+          // This operation will not be exposed to a non-isolated context. In such a context,
+          // there will be no "calculateSecretResult" property on PowerfulFeature.prototype.
+          [CrossOriginIsolated] Promise&lt;Result&gt; calculateSecretResult();
+
+          // The same applies here: the attribute will not be exposed to a non-isolated context,
+          // and in a non-secure context there will be no "secretBoolean" property on
+          // PowerfulFeature.prototype.

```suggestion
          // ExampleFeature.prototype.
```

> +          // and in a non-secure context there will be no "secretBoolean" property on
+          // PowerfulFeature.prototype.
+          [CrossOriginIsolated] readonly attribute boolean secretBoolean;
+        };
+
+        // HighResolutionTimer will not be exposed in a non-isolated context, nor will its members.
+        // In such a context, there will be no "HighResolutionTimer" property on Window.
+        [CrossOriginIsolated]
+        interface HighResolutionTimer {
+          DOMHighResTimeStamp getHighResolutionTime();
+        };
+
+        // The interface mixin members defined below will never be exposed in a non-isolated context,
+        // regardless of whether the interface that includes them is.
+        // In a non-isolated context, there will be no "snap" property on
+        // PowerfulFeature.prototype.

```suggestion
        // ExampleFeature.prototype.
```

> +        // HighResolutionTimer will not be exposed in a non-isolated context, nor will its members.
+        // In such a context, there will be no "HighResolutionTimer" property on Window.
+        [CrossOriginIsolated]
+        interface HighResolutionTimer {
+          DOMHighResTimeStamp getHighResolutionTime();
+        };
+
+        // The interface mixin members defined below will never be exposed in a non-isolated context,
+        // regardless of whether the interface that includes them is.
+        // In a non-isolated context, there will be no "snap" property on
+        // PowerfulFeature.prototype.
+        [CrossOriginIsolated]
+        interface mixin Snapshotable {
+          Promise&lt;boolean&gt; snap();
+        };
+        PowerfulFeature includes Snapshotable;

```suggestion
        ExampleFeature includes Snapshotable;
```

> +
+        // The interface mixin members defined below will never be exposed in a non-isolated context,
+        // regardless of whether the interface that includes them is.
+        // In a non-isolated context, there will be no "snap" property on
+        // PowerfulFeature.prototype.
+        [CrossOriginIsolated]
+        interface mixin Snapshotable {
+          Promise&lt;boolean&gt; snap();
+        };
+        PowerfulFeature includes Snapshotable;
+
+        // On the other hand, the following interface mixin members will be exposed
+        // to a non-isolated context when included by a host interface
+        // that doesn't have the [CrossOriginIsolated] extended attribute.
+        // In a non-secure context, there will be a "log" property on
+        // PowerfulFeatures.prototype.

```suggestion
        // ExampleFeatures.prototype.
```

> +        // PowerfulFeature.prototype.
+        [CrossOriginIsolated]
+        interface mixin Snapshotable {
+          Promise&lt;boolean&gt; snap();
+        };
+        PowerfulFeature includes Snapshotable;
+
+        // On the other hand, the following interface mixin members will be exposed
+        // to a non-isolated context when included by a host interface
+        // that doesn't have the [CrossOriginIsolated] extended attribute.
+        // In a non-secure context, there will be a "log" property on
+        // PowerfulFeatures.prototype.
+        interface mixin Loggable {
+          Promise&lt;boolean&gt; log();
+        };
+        PowerfulFeatures includes Loggable;

```suggestion
        ExampleFeatures includes Loggable;
```

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