Here is my solution. Any comments / thoughts are welcome.

// declare program variables
var num1;
var num2;
var message;

// announce the program
alert("Let's do some math!");


// loop until condition 1 is correct
while(isNaN(num1) || num1 === 0){

  // collect numeric input 1
  num1 = prompt("Please type a number");
  num1 = parseFloat(num1);

    // check input 1
    if(isNaN(num1) || num1 === 0){
      alert("that is not a valid input, try again")
    }
}
// loop until conditoon 2 is correct
while(isNaN(num2) || num2 === 0){

  // collect numeric input 2
  num2 = prompt("Please type another number");
  num2 = parseFloat(num2);
    // check input 2
    if(isNaN(num2) || num2 === 0){
      alert("that is not a valid input, try again")
    }
}

// build an HTML message
message = "<h1>Math with the numbers " + num1 + " and " + num2 + "</h1>";
message += num1 + " + " + num2 + " = " + (num1 + num2);
message += "<br>";
message += num1 + " * " + num2 + " = " + (num1 * num2);
message += "<br>";
message += num1 + " / " + num2 + " = " + (num1 / num2);
message += "<br>";
message += num1 + " - " + num2 + " = " + (num1 - num2);

// write message to web page
document.write(message);