See the Pen Login form by TszHong (@tszhong0411) on CodePen.

HTML

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<div class="login-box">
<h2>Login</h2>
<form>
<div class="user-box">
<input type="text" name="" required>
<label>Username</label>
</div>
<div class="user-box">
<input type="password" name="" required>
<label>Password</label>
</div>
<a href="#">
<span></span>
<span></span>
<span></span>
<span></span>
Submit
</a>
</form>
</div>

CSS

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html {
height: 100%;
}
body {
margin:0;
padding:0;
font-family: sans-serif;
background: linear-gradient(#141e30, #243b55);
}

.login-box {
position: absolute;
top: 50%;
left: 50%;
width: 400px;
padding: 40px;
transform: translate(-50%, -50%);
background: rgba(0,0,0,.5);
box-sizing: border-box;
box-shadow: 0 15px 25px rgba(0,0,0,.6);
border-radius: 10px;
}

.login-box h2 {
margin: 0 0 30px;
padding: 0;
color: #fff;
text-align: center;
}

.login-box .user-box {
position: relative;
}

.login-box .user-box input {
width: 100%;
padding: 10px 0;
font-size: 16px;
color: #fff;
margin-bottom: 30px;
border: none;
border-bottom: 1px solid #fff;
outline: none;
background: transparent;
}
.login-box .user-box label {
position: absolute;
top:0;
left: 0;
padding: 10px 0;
font-size: 16px;
color: #fff;
pointer-events: none;
transition: .5s;
}

.login-box .user-box input:focus ~ label,
.login-box .user-box input:valid ~ label {
top: -20px;
left: 0;
color: #03e9f4;
font-size: 12px;
}

.login-box form a {
position: relative;
display: inline-block;
padding: 10px 20px;
color: #03e9f4;
font-size: 16px;
text-decoration: none;
text-transform: uppercase;
overflow: hidden;
transition: .5s;
margin-top: 40px;
letter-spacing: 4px
}

.login-box a:hover {
background: #03e9f4;
color: #fff;
border-radius: 5px;
box-shadow: 0 0 5px #03e9f4,
0 0 25px #03e9f4,
0 0 50px #03e9f4,
0 0 100px #03e9f4;
}

.login-box a span {
position: absolute;
display: block;
}

.login-box a span:nth-child(1) {
top: 0;
left: -100%;
width: 100%;
height: 2px;
background: linear-gradient(90deg, transparent, #03e9f4);
animation: btn-anim1 1s linear infinite;
}

@keyframes btn-anim1 {
0% {
left: -100%;
}
50%,100% {
left: 100%;
}
}

.login-box a span:nth-child(2) {
top: -100%;
right: 0;
width: 2px;
height: 100%;
background: linear-gradient(180deg, transparent, #03e9f4);
animation: btn-anim2 1s linear infinite;
animation-delay: .25s
}

@keyframes btn-anim2 {
0% {
top: -100%;
}
50%,100% {
top: 100%;
}
}

.login-box a span:nth-child(3) {
bottom: 0;
right: -100%;
width: 100%;
height: 2px;
background: linear-gradient(270deg, transparent, #03e9f4);
animation: btn-anim3 1s linear infinite;
animation-delay: .5s
}

@keyframes btn-anim3 {
0% {
right: -100%;
}
50%,100% {
right: 100%;
}
}

.login-box a span:nth-child(4) {
bottom: -100%;
left: 0;
width: 2px;
height: 100%;
background: linear-gradient(360deg, transparent, #03e9f4);
animation: btn-anim4 1s linear infinite;
animation-delay: .75s
}

@keyframes btn-anim4 {
0% {
bottom: -100%;
}
50%,100% {
bottom: 100%;
}
}