diff --git a/HISTORY.md b/HISTORY.md
index 942a240b..69814dcd 100755
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -10,6 +10,9 @@
 * **Notable Fixes:**
     * fixed occasional invisible error messages
 
+### 2021-06-25
+* extensions: added first rough experimental version of web-serial extension primitives
+
 ### 2021-06-24
 * extensions: tweaked loading unlisted script-extensions
 * byob, threads, store: removed unused code
diff --git a/snap.html b/snap.html
index 42a950dd..0b0034c9 100755
--- a/snap.html
+++ b/snap.html
@@ -19,7 +19,7 @@
         <script src="src/sketch.js?version=2020-07-13"></script>
         <script src="src/video.js?version=2019-06-27"></script>
         <script src="src/maps.js?version=2021-06-15"></script>
-        <script src="src/extensions.js?version=2021-06-24"></script>
+        <script src="src/extensions.js?version=2021-06-25"></script>
         <script src="src/xml.js?version=2020-04-27"></script>
         <script src="src/store.js?version=2021-06-24"></script>
         <script src="src/locale.js?version=2021-06-11"></script>
diff --git a/src/extensions.js b/src/extensions.js
index 0ffbee87..4f3b71c8 100644
--- a/src/extensions.js
+++ b/src/extensions.js
@@ -31,7 +31,7 @@
 IDE_Morph, CamSnapshotDialogMorph, SoundRecorderDialogMorph, isSnapObject, nop,
 Color, Process, contains*/
 
-modules.extensions = '2021-June-24';
+modules.extensions = '2021-June-25';
 
 // Global stuff
 
@@ -716,6 +716,107 @@ SnapExtensions.primitives.set(
     }
 );
 
+// web serial (srl_): // +++ under construction
+
+SnapExtensions.primitives.set(
+    'srl_open',
+    function () {
+
+        var world = this.world(); // +++ change to IDE
+
+        async function webSerialConnect() { // +++ instead of async make it Snap! thread friendly
+            // Prompt user to choose a serial port and open the one selected.
+
+            var vendorIDs = [
+                {usbVendorId: 0x0403},		// FTDI
+                {usbVendorId: 0x0d28},		// micro:bit, Calliope
+                {usbVendorId: 0x10c4},		// Silicon Laboratories, Inc. (CP210x)
+                {usbVendorId: 0x1a86},		// CH340
+                {usbVendorId: 0x239a},		// AdaFruit
+                {usbVendorId: 0x2a03},		// Arduino
+                {usbVendorId: 0x2341},		// Arduino MKR Zero
+                {usbVendorId: 0x03eb},		// Atmel Corporation
+                {usbVendorId: 0x1366},		// SEGGER Calliope mini
+                {usbVendorId: 0x16c0},		// Teensy
+            ];
+            world.webSerialPort = await navigator.serial.requestPort({filters: vendorIDs}).catch((e) => { console.log(e); }); // +++ make "await" Snap! thread friendly, change UI if possible to a non-blocking Morphic menu/dialog
+            if (!world.webSerialPort) {return; } // no serial port selected
+            await world.webSerialPort.open({ baudRate: 115200 }); // +++ make "await" Snap! thread friendly
+            world.webSerialReader = await world.webSerialPort.readable.getReader(); // +++ make "await" Snap! thread friendly
+            webSerialReadLoop();
+        }
+
+        async function webSerialReadLoop() {
+            try {
+                while (true) { // +++ should be improved, make thread-friendly
+                    var {value, done} = await world.webSerialReader.read(); // +++ make "await" Snap! thread friendly
+                    if (value) {
+                        world.serialInputBuffers.push(Array.from(value));
+                    }
+                    if (done) { // happens when world.webSerialReader.cancel() is called by disconnect
+                        world.webSerialReader.releaseLock();
+                        return;
+                    }
+                }
+            } catch (e) { // happens when board is unplugged
+                // console.log(e);
+                if (world.webSerialPort) await world.webSerialPort.close().catch(() => {}); // +++ make "await" Snap! thread friendly
+                world.webSerialPort = null; // +++ reformulate "world", use IDE reference
+                world.webSerialReader = null;
+            }
+        }
+
+        world.serialInputBuffers = [];
+        webSerialConnect();
+    }
+);
+
+SnapExtensions.primitives.set(
+    'srl_close',
+    function () {
+        var world = this.world(); // +++ change to IDE
+
+        async function webSerialDisconnect() { // +++ make async thread friendly
+            if (world.webSerialReader) await world.webSerialReader.cancel(); // +++ make "await" Snap! thread friendly
+            if (world.webSerialPort) await world.webSerialPort.close().catch(() => {}); // +++ make "await" Snap! thread friendly
+            world.webSerialReader = null;
+            world.webSerialPort = null;
+        }
+
+        webSerialDisconnect();
+    }
+);
+
+SnapExtensions.primitives.set(
+    'srl_read',
+    function () {
+        var world = this.world(); // +++ change to IDE
+
+        function webSerialRead() { // +++ do we need this function at all? Why not run this code directly?
+            if (world.serialInputBuffers && (world.serialInputBuffers.length > 0)) {
+                var result = [].concat.apply([], world.serialInputBuffers);
+                world.serialInputBuffers = [];
+                return new List(result);
+            }
+            return '';
+        }
+
+        return webSerialRead();
+    }
+);
+
+SnapExtensions.primitives.set(
+    'srl_write(dta)',
+    function (data) {
+        var world = this.world(); // +++ change to IDE
+
+        if (!world.webSerialPort || !world.webSerialPort.writable) return 0;  // port not open
+        const w = world.webSerialPort.writable.getWriter();
+        w.write(data.buffer);
+        w.releaseLock();
+    }
+);
+
 // loading external scripts (src_)
 
 SnapExtensions.primitives.set(