, пожалуйста, используйте следующий код, я внес изменения в ваш HTML, обернув элементы в DIVs и изменив ваш CSS
<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="//code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
<script type="text/javascript" src="https://cdn.jsdelivr.net/npm/jquery-
validation@1.17.0/dist/jquery.validate.js"></script>
<style>
</style>
</head>
<body>
<form class="form-panel" id="signupForm">
<div class="form-fname">
<input type="text" name="fname" placeholder="First Name">
</div>
<div class="form-lname">
<input type="text" name="lname" placeholder="Last Name">
<br>
</div>
<div class="form-email">
<input type="email" name="email" placeholder="Email Address">
<br>
</div>
<div class="form-password">
<input type="password" name="password" placeholder="Password">
</div>
<div class="form-password-confirm">
<input type="password" name="cpassword" placeholder="Comfirm Password">
</div>
<input type="submit" class="btn btn-default submit-button" value="Sign up!">
</form>
<script>
</script>
</body>
</html>
Код CSS
.form-panel {
margin-left: auto;
margin-right: auto;
width: 60%;
position : relative;
display : block;
}
input[type="text"],
input[type="password"] {
padding: 4px 7px;
height: 35px;
border: 1px solid black;
border-radius: 3px;
position: relative;
width: 89%;
display: block;
}
input[type="email"]{
padding: 4px 0px 4px 5px;
height: 35px;
border: 1px solid black;
border-radius: 3px;
width: 98%;
display: block;
position: relative;
}
/*First name row*/
.form-fname{
display : inline-block;
position : relative;
width : 45%;
}
/*Last name row*/
.form-lname{
display : inline-block;
position : relative;
width : 45%;
float : right;
}
/*Email row*/
.form-email{
display : block;
position : relative;
width : 100%;
}
/*Password row*/
.form-password{
display : inline-block;
position : relative;
width : 45%;
}
/*Password Confirm row*/
.form-password-confirm{
display : inline-block;
position : relative;
width : 45%;
float : right;
}
label{
display : block;
position : relative;
width : 100%;
margin-top : 2px;
margin-bottom : 5px
}
.submit-button{
margin-top : 6px
}