The website layout is not fixed

folks, I am learning the basics of web development. I have used two fieldsets in the page as CSS id

#lfieldset
{
    width: 1019px;
    height: 500px;
    background: #ffffff;
    border-radius: 8px;
    border: none;
    float: left;
}
#rfieldset
{
    width: 300px;
    height: 200px;
    background: #ffffff;
    border-radius: 8px;
    border: none;
    float: right;
}

this is how it looks

Screenshot1

and the meta tag

<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">

for layout, but the second fieldset comes to the bottom if I reduce the window, see

Screenshot2

how can I solve this?