Add Explainer for On-Device Speech Recognition

ondevice/explainer.md

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+# Explainer: On-Device Speech Recognition for the Web Speech API
+
+## Introduction
+
+The Web Speech API is a powerful browser feature that enables applications to perform speech recognition. Traditionally, this functionality relies on sending audio data to cloud-based services for recognition. While this approach is effective, it has certain drawbacks:
+
+- **Privacy concerns:** Raw and transcribed audio is transmitted over the network.
+- **Latency issues:** Users may experience delays due to network communication.
+- **Offline limitations:** Speech recognition does not work without an internet connection.
+
+To address these issues, we introduce **on-device speech recognition capabilities** as part of the Web Speech API. This enhancement allows speech recognition to run locally on user devices, providing a faster, more private, and offline-compatible experience.
+
+## Why Use On-Device Speech Recognition?
+
+### 1. **Privacy**
+On-device processing ensures that neither raw audio nor transcriptions leave the user's device, enhancing data security and user trust.
+
+### 2. **Performance**
+Local processing reduces latency, providing a smoother and faster user experience.
+
+### 3. **Offline Functionality**
+Applications can offer speech recognition capabilities even without an active internet connection, increasing their utility in remote or low-connectivity environments.
+
+## New Methods
+
+### 1. `Promise<boolean> onDeviceWebSpeechAvailable(DOMString lang)`
+This method checks if on-device speech recognition is available for a specific language. Developers can use this to determine whether to enable on-device features or fall back to cloud-based recognition.
+
+#### Example Usage
+```javascript
+const lang = 'en-US';
+SpeechRecognition.onDeviceWebSpeechAvailable(lang).then((available) => {
+    if (available) {
+        console.log(`On-device speech recognition is available for ${lang}.`);
+    } else {
+        console.log(`On-device speech recognition is not available for ${lang}.`);
+    }
+});
+```
+
+### 2. `Promise<boolean> installOnDeviceSpeechRecognition()`
+This method initiates the installation of resources required for on-device speech recognition. The installation process may download and configure necessary language models.
+
+#### Example Usage
+```javascript
+SpeechRecognition.installOnDeviceSpeechRecognition().then(() => {
+    console.log('Installation of on-device speech recognition resources initiated successfully.');
+}).catch((error) => {
+    console.error('Unable to install on-device speech recognition:', error);
+});
+```
+
+## New Attribute
+
+### 1. `mode` attribute in the `SpeechRecognition` interface
+The `mode` attribute in the `SpeechRecognition` interface defines how speech recognition should behave when starting a session.
+
+#### `SpeechRecognitionMode` Enum
+
+- **"on-device-preferred"**: Use on-device speech recognition if available. If not, fall back to cloud-based speech recognition.
+- **"on-device-only"**: Only use on-device speech recognition. If it's unavailable, throw an error.
+- **"cloud-only"**: Only use cloud-based speech recognition, bypassing on-device options entirely.
+
+#### Example Usage
+```javascript
+const recognition = new SpeechRecognition();
+recognition.mode = "ondevice-only"; // Only use on-device speech recognition.
+recognition.start();
+```
+
+## Privacy considerations
+To reduce the risk of fingerprinting, user agents must obtain explicit and informed user consent before installing on-device speech recognition languages that differ from the user's preferred language or when the user is not connected to an Ethernet or Wi-Fi network.
+
+## Conclusion
+
+The addition of on-device speech recognition capabilities to the Web Speech API marks a significant step forward in creating more private, performant, and accessible web applications. By leveraging these new methods, developers can enhance user experiences while addressing key concerns around privacy and connectivity.