not putting a = in a for loop hanged my laptop

Hello I’m new to programming so please don’t mind my silly questions. And please help me!

So, I was learning loops and started with for loop. I wanted to display :

1
11
21
31
41
51
61
71
81
91

(10 getting added to 1)

So I was referring a youtube video and it suggested the following way :

<html>
  <title>I'm Learning</title>
</html>
<body>
  <script>
    for (var a = 1; a<=100;a = a+10){
        document.write(a + "<br>")
    }
  </script>
</body>
</html>

Which worked.enter image description here But, when I removed a = from for (var a = 1; a<=100;a = a+10) I thought it should still work as the value of a will still get changed. But, the webpage kept loading and after a while my laptop hanged. This happened twice. The webpage did not even load! So can anyone please help me with this? Please explain me if possible that why this error occurred. Thank You.

I was expecting that the output would not change.