Enter the following line in your chrome (browser) address bar

chrome://flags/#enable-experimental-web-platform-features

Click the Disabled dropdown for Experimental Web Platform features and select Enabled

*https://www.chromestatus.com/feature/6577673212002304 

I am curious about the firefox skepticism

Use this code to prepare your Arduino for serial communication with the browser:

String incoming = "";
void setup()
{
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.begin(9600);
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only
  } 
  Serial.println("Serial is READY");
}


void loop()
{
  if (Serial.available() > 0) {
    incoming = Serial.readString();    
    delay(5);

    if (incoming == "LON" || incoming == "1") {
      digitalWrite(LED_BUILTIN, HIGH);
      Serial.println("LED is ON");
    }

    if (incoming == "LOFF" || incoming == "0") {
      digitalWrite(LED_BUILTIN, LOW);
      Serial.println("LED is OFF");
    }
    Serial.print("Serial READ: ");
    Serial.println(incoming);
  }
  //delay(10);
  
    
}

 You will need to use a current Arduino IDE or other methods to flash this to your board at this time but feel free to make your own customization.

Check out https://hello-arduino-from-chrome.glitch.me/ to test the functionality. You can even remix your own interface from there.

Connect your device to the computer and click connect in the browser.

Based on the code we uploaded to our Arduino we should see that our device is ready. We can send "LON" or a 1 to turn on the built in LED and send "LOFF" or 0 to turn it off (no quotes when you enter those commands) 

At the very least you can send a "Hello" and see that it responds.