WordPress – unnecessary bottom margin is being added to the div

In my wp site, there is a registration page. I have placed a reg form in it.

Page link –> Reg Page

the form was placed in a div which contains class .form-contain.

.form-contain {
    display: flex;
    align-items: center;
    border: 2px solid #ccc;
}

<div class="form-contain">
  <form> </form>
</div>

I have not added any margin (not even margin-bottom), but extra space is appearing after the form-contain div.

why it is adding and how to remove this extra space ?

for that i have tried the following, but it was not worked.

html, body {
    margin: 0;
    padding: 0;
}