the input is inside the fildset tag, but it is cutting the line

You probably set the width to 100% then try adding:

input {
    outline: 0;
    margin: 0;
    box-sizing: border-box !important;
}

And if you still do not use a reset:

* {
    margin: 0;
    padding: 0;
}

Good evening. It is normal for this kind of thing to happen, especially with inputs. Usually, we have to define his style in the hand, to be both aesthetically better, in terms of bugs and etc ... In your case, it is only to decrease the width of the element that best suits your taste.

• Note that changing the INPUT attribute will all have the same dimensions.

So maybe this will solve your problem:

input {      
font-size: 16px;
margin-bottom: 20px;
border-radius: 5px;
border: 1px solid #ccc;
position: relative;
width: 90%; /* Esta será a alteração que fará a diferença */
}