--- a/dom/base/nsScriptLoader.cpp
+++ b/dom/base/nsScriptLoader.cpp
@@ -71,16 +71,19 @@ static LazyLogModule gScriptLoaderLog("S
   MOZ_LOG(gScriptLoaderLog, mozilla::LogLevel::Verbose, args)
 #define LOG(args)                                                             \
   MOZ_LOG(gScriptLoaderLog, mozilla::LogLevel::Debug, args)
 #define LOG_WARN(args)                                                        \
   MOZ_LOG(gScriptLoaderLog, mozilla::LogLevel::Warning, args)
 #define LOG_ERROR(args)                                                       \
   MOZ_LOG(gScriptLoaderLog, mozilla::LogLevel::Error, args)
 
+// Measuring DOM_SCRIPT_SRC_UNICODE_SUBSET can be expensive, so we only do it
+// randomly, on average once every SAMPLE_DOM_SCRIPT_SRC_UNICODE_SUBSET times.
+#define SAMPLE_DOM_SCRIPT_SRC_UNICODE_SUBSET 10000
 
 void
 ImplCycleCollectionUnlink(nsScriptLoadRequestList& aField);
 
 void
 ImplCycleCollectionTraverse(nsCycleCollectionTraversalCallback& aCallback,
                             nsScriptLoadRequestList& aField,
                             const char* aName,
@@ -3086,12 +3089,30 @@ nsScriptLoadHandler::OnStreamComplete(ns
                                            /* aEndOfStream = */ true);
 
     // If SRI is required for this load, appending new bytes to the hash.
     if (mSRIDataVerifier && NS_SUCCEEDED(mSRIStatus)) {
       mSRIStatus = mSRIDataVerifier->Update(aDataLength, aData);
     }
   }
 
+  // Update Telemetry.
+  // As this is expensive, we only do it on a randomized sample of scripts.
+  if (Telemetry::CanRecordExtended() && (rand() % SAMPLE_DOM_SCRIPT_SRC_UNICODE_SUBSET == 0)) {
+    const mozilla::TimeStamp start = mozilla::TimeStamp::Now();
+    bool isSingleByte = true;
+    for (char16_t c: mRequest->mScriptText) {
+      if (c < 0xFF) {
+        isSingleByte = false;
+        break;
+      }
+    }
+    const mozilla::TimeStamp stop = mozilla::TimeStamp::Now();
+    Telemetry::Accumulate(Telemetry::DOM_SCRIPT_SRC_UNICODE_SUBSET,
+        isSingleByte ? 1 /* "Latin 1" */: 0 /* Anything else */);
+    Telemetry::Accumulate(Telemetry::DOM_SCRIPT_SRC_UNICODE_SUBSET_DURATION_MS,
+        (stop - start).ToMilliseconds());
+  }
+
   // we have to mediate and use mRequest.
   return mScriptLoader->OnStreamComplete(aLoader, mRequest, aStatus, mSRIStatus,
                                          mSRIDataVerifier);
 }

--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -10543,16 +10543,33 @@
   "DOM_SCRIPT_SRC_ENCODING": {
     "alert_emails": ["dteller@mozilla.com"],
     "expires_in_version": "never",
     "kind": "count",
     "keyed": true,
     "bug_numbers": [1344152],
     "description": "Encoding used in a <script src>."
   },
+  "DOM_SCRIPT_SRC_UNICODE_SUBSET": {
+    "alert_emails": ["dteller@mozilla.com"],
+    "expires_in_version": "never",
+    "kind": "enumerated",
+    "bug_numbers": [1344152],
+    "n_values": 10,
+    "description": "Actual subset of Unicode used in a <script src>. Current values are 0 (unspecified), 1 (all chars are < 0xff)."
+  },
+  "DOM_SCRIPT_SRC_UNICODE_SUBSET_DURATION_MS": {
+    "alert_emails": ["dteller@mozilla.com"],
+    "expires_in_version": "never",
+    "kind": "exponential",
+    "bug_numbers": [1344152],
+    "n_buckets": 10,
+    "high": 10000,
+    "description": "Time spent collecting the information to find out the value of DOM_SCRIPT_SRC_UNICODE_SUBSET, in ms."
+  },
   "WEB_NOTIFICATION_REQUEST_PERMISSION_CALLBACK": {
     "alert_emails": ["push@mozilla.com"],
     "expires_in_version": "55",
     "bug_numbers": [1241278],
     "kind": "boolean",
     "description": "Usage of the deprecated Notification.requestPermission() callback argument"
   },
   "VIDEO_FASTSEEK_USED": {