Add Explainer for SpeechRecognitionResult timestamps

explainers/speech-recognition-result-timestamps‎

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+# Web Speech API Proposal: SpeechRecognitionResult Timestamps
+
+**Author:** alanding@google.com, evliu@google.com
+
+### Problem
+
+The Web Speech API currently does not expose the start and end timestamps of the source audio corresponding to a given transcription result (`SpeechRecognitionResult`). This limitation creates two major challenges for API clients and end users:
+
+- **Timeline Association:** Developers cannot readily associate transcribed text with specific segments of the audio source, making it difficult to map generated captions to media timelines or audio tracks.
+- **Latency Tracking & Backend Failover:** With the adoption of on-device Automatic Speech Recognition (ASR) to improve privacy and reduce server costs, processing performance becomes heavily dependent on local client hardware resources. The Web Speech API acts as a "black box" regarding local processing delays. Developers cannot programmatically calculate transcription latency or detect when on-device models fall behind real-time. This leads to poor user experiences (e.g. caption lag during live video conferencing) and deprives applications of the signal needed to seamlessly fail over to high-performance cloud backends.
+
+### Proposed Solution
+
+We propose extending the `SpeechRecognitionResult` interface to include optional (nullable) `audioStartTime` and `audioEndTime` attributes.
+
+#### Web IDL Definition
+
+```webidl
+partial interface SpeechRecognitionResult {
+    // Start timestamp of the audio segment in milliseconds (relative to time origin)
+    readonly attribute DOMHighResTimeStamp? audioStartTime;
+
+    // End timestamp of the audio segment in milliseconds (relative to time origin)
+    readonly attribute DOMHighResTimeStamp? audioEndTime;
+};
+```
+
+### Proposed Behavior & Example Usage
+
+The `audioStartTime` and `audioEndTime` properties represent the audio duration bounds (in milliseconds) corresponding to the transcribed segment. If the underlying recognition engine backend does not support segment timestamps, these attributes return `null`.
+
+Developers can programmatically compute processing latency by comparing `audioEndTime` against the standard DOM event generation timestamp (`Event.timeStamp`):
+
+```javascript
+const recognition = new SpeechRecognition();
+recognition.continuous = true;
+recognition.interimResults = true;
+
+recognition.onresult = (event) => {
+  const result = event.results[event.resultIndex];
+
+  if (result.audioEndTime !== null) {
+    // Calculate on-device processing latency
+    const processingLatencyMs = event.timeStamp - result.audioEndTime;
+
+    // Trigger seamless failover to cloud backend if latency breaches acceptable threshold
+    if (processingLatencyMs > 1500) {
+      console.warn(`ASR processing lag detected (${processingLatencyMs}ms). Transitioning to cloud provider.`);
+      switchToCloudBackend();
+    }
+  }
+};
+
+recognition.start();
+```
+
+### Security and Privacy Considerations
+
+#### Fingerprinting Risk
+Exposing sub-millisecond or precise micro-architectural timing information enables hardware profiling (measuring CPU execution speed, thermal throttling, and system load), creating a tracking vector for cross-origin user fingerprinting.
+
+#### Mitigation Strategy
+To mitigate fingerprinting vectors, browser implementations MUST apply timestamp fuzzing and precision reduction before exposing timing attributes to web scripts. We propose mirroring the precision capping strategy used in [`HTMLMediaElement.currentTime`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/currentTime), rounding exposed timestamps to **2ms** precision (or matching the site-wide timer resolution policy).
+
+### Alternatives Considered
+
+- **Browser-Generated Warning Events (`onprocessinglag`):** Simple for web applications to catch, but fails to accommodate varying latency thresholds across different use cases (e.g. real-time meeting captioning requires <200ms latency, while dictation tools tolerate multi-second delays).
+- **Internal Processing Queue Metric (`queueDepth`):** Directly exposes engine backlogs, but is difficult to standardize across fragmented engine architectures, model types, and buffering strategies.
+- **Binary Status Flag (`isRealTime`):** Simple boolean check, but lacks numerical precision for applications seeking to track progressive latency degradation trendlines.