Animated background Login UI | HTML, CSS & JavaScript

<div class="container">
    
    <div class="triangle-frame"></div>
    <div class="flip-container">
        <div class="flip-card" id="flipCard">
            <div class="form-card">
                <h1>Login</h1>
                <form id="loginForm">
                    <div class="input-box">
                        <input
                            type="text"
                            id="username"
                            placeholder=" "
                        >
                        <label>Username</label>
                    </div>
                    <div class="input-box">
                        <input
                            type="password"
                            id="password"
                            placeholder=" "
                        >
                        <label>Password</label>
                    </div>
                    <button type="submit">
                        SIGN IN
                    </button>
                </form>
                <div
                    id="loginMessage"
                    class="message"
                ></div>
                <div class="links">
                    <a href="#">
                        Forgot Password?
                    </a>
                    <br><br><br>
                    <a id="showSignup">
                        Create New Account
                    </a>
                </div>
            </div>

<div class="form-card signup-card">
                <h1>Sign Up</h1>
                <form id="signupForm">
                    <div class="input-box">
                        <input
                            type="text"
                            id="signupName"
                            placeholder=" "
                        >
                        <label>Full Name</label>
                    </div>
                    <div class="input-box">
                        <input
                            type="email"
                            id="signupEmail"
                            placeholder=" "
                        >
                        <label>Email</label>
                    </div>
                    <div class="input-box">
                        <input
                            type="password"
                            id="signupPassword"
                            placeholder=" "
                        >
                        <label>Password</label>
                    </div>
                    <button type="submit">
                        CREATE ACCOUNT
                    </button>
                </form>
                <div
                    id="signupMessage"
                    class="message"
                ></div>
                <div class="links">
                    <a id="showLogin">
                        ← Back to Login
                    </a>
                </div>
            </div>
        </div>
    </div>
</div>

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    min-height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    overflow: hidden;
    font-family: Arial, sans-serif;
    background:
        radial-gradient(circle at 20% 20%, #46127a 0%, transparent 30%),
        radial-gradient(circle at 80% 80%, #b41470 0%, transparent 30%),
        linear-gradient(135deg, #10001f, #23003b, #10001f);
}

/* =========================
   BACKGROUND ANIMATION
========================= */

.background {
    position: fixed;
    inset: 0;
    overflow: hidden;
    z-index: 0;
}

/* Large rotating triangle */

.bg-triangle {
    position: absolute;
    width: 650px;
    height: 650px;

left: 50%;
    top: 50%;

transform: translate(-50%, -50%);

clip-path: polygon(
        50% 0%,
        100% 100%,
        0% 100%
    );

background:
        linear-gradient(
            45deg,
            #ff0080,
            #7928ca,
            #ff0080,
            #00e5ff
        );

background-size: 400% 400%;

opacity: 0.25;

filter: blur(2px);

animation:
        backgroundTriangleRotate 18s linear infinite,
        backgroundGradient 8s ease infinite;
}

/* Second triangle */

.bg-triangle.two {
    width: 900px;
    height: 900px;

opacity: 0.10;

animation:
        backgroundTriangleReverse 25s linear infinite,
        backgroundGradient 12s ease infinite;
}

/* Triangle glow */

.bg-triangle::before {
    content: "";
    position: absolute;

inset: 15px;

clip-path: polygon(
        50% 0%,
        100% 100%,
        0% 100%
    );

background: #12001f;
}

/* Rotating background */

@keyframes backgroundTriangleRotate {

0% {
        transform:
            translate(-50%, -50%)
            rotate(0deg);
    }

100% {
        transform:
            translate(-50%, -50%)
            rotate(360deg);
    }
}

@keyframes backgroundTriangleReverse {

0% {
        transform:
            translate(-50%, -50%)
            rotate(360deg);
    }

100% {
        transform:
            translate(-50%, -50%)
            rotate(0deg);
    }
}

@keyframes backgroundGradient {

0% {
        background-position: 0% 50%;
    }

50% {
        background-position: 100% 50%;
    }

100% {
        background-position: 0% 50%;
    }
}

/* =========================
   FLOATING PARTICLES
========================= */

.particle {
    position: absolute;

width: 5px;
    height: 5px;

border-radius: 50%;

background: #ff4da6;

box-shadow:
        0 0 10px #ff0080,
        0 0 20px #ff0080;

animation: floatParticle 8s linear infinite;
}

.p1 {
    left: 10%;
    bottom: -20px;
    animation-delay: 0s;
}

.p2 {
    left: 25%;
    bottom: -20px;
    animation-delay: 2s;
}

.p3 {
    left: 45%;
    bottom: -20px;
    animation-delay: 4s;
}

.p4 {
    left: 70%;
    bottom: -20px;
    animation-delay: 1s;
}

.p5 {
    left: 90%;
    bottom: -20px;
    animation-delay: 3s;
}

@keyframes floatParticle {

0% {
        transform: translateY(0);
        opacity: 0;
    }

20% {
        opacity: 1;
    }

100% {
        transform: translateY(-110vh);
        opacity: 0;
    }
}

/* =========================
   MAIN CONTAINER
========================= */

.container {
    position: relative;

width: 420px;
    min-height: 520px;

display: flex;
    justify-content: center;
    align-items: center;

z-index: 10;
}

/* =========================
   STATIC NEON TRIANGLE FRAME
========================= */

.triangle-frame {
    position: absolute;

width: 410px;
    height: 410px;

top: 40px;

clip-path: polygon(
        50% 0%,
        100% 100%,
        0% 100%
    );

background:
        linear-gradient(
            135deg,
            #ff0080,
            #7928ca,
            #ff0080
        );

filter:
        drop-shadow(0 0 8px #ff0080)
        drop-shadow(0 0 25px #8b2cff);

animation: frameGlow 3s ease-in-out infinite;
}

.triangle-frame::before {
    content: "";

position: absolute;

inset: 5px;

clip-path: polygon(
        50% 0%,
        100% 100%,
        0% 100%
    );

background: #180022;
}

@keyframes frameGlow {

0%,
    100% {
        filter:
            drop-shadow(0 0 6px #ff0080)
            drop-shadow(0 0 15px #7928ca);
    }

50% {
        filter:
            drop-shadow(0 0 15px #ff0080)
            drop-shadow(0 0 35px #7928ca);
    }
}

/* =========================
   LOGIN / SIGNUP CARD
========================= */

.flip-container {
    position: relative;

width: 320px;
    height: 390px;

perspective: 1000px;

z-index: 20;
}

.flip-card {
    position: relative;

width: 100%;
    height: 100%;

transform-style: preserve-3d;

transition: transform 0.8s ease;
}

.flip-card.flipped {
    transform: rotateY(180deg);
}

.form-card {
    position: absolute;

width: 100%;
    height: 100%;

padding: 38px 30px;

border-radius: 20px;

background:
        linear-gradient(
            145deg,
            rgba(45, 0, 70, 0.95),
            rgba(18, 0, 32, 0.98)
        );

border: 1px solid rgba(255, 0, 128, 0.5);

box-shadow:
        10px 10px 30px rgba(0,0,0,0.6),
        -5px -5px 20px rgba(255,0,128,0.08),
        inset 0 0 20px rgba(255,0,128,0.08);

backface-visibility: hidden;
}

.signup-card {
    transform: rotateY(180deg);
}

/* =========================
   FORM
========================= */

h1 {
    text-align: center;

margin-bottom: 28px;

color: #fff;

font-size: 30px;

text-shadow:
        0 0 10px #ff0080,
        0 0 25px #7928ca;
}

.input-box {
    position: relative;

margin-bottom: 22px;
}

.input-box input {
    width: 100%;

padding: 14px 15px;

border: none;
    outline: none;

border-radius: 10px;

background: rgba(255,255,255,0.06);

border: 1px solid rgba(255,0,128,0.35);

color: #fff;

font-size: 14px;

transition: 0.3s;
}

.input-box input:focus {
    border-color: #ff0080;

box-shadow:
        0 0 8px rgba(255,0,128,0.5),
        0 0 20px rgba(121,40,202,0.3);
}

.input-box label {
    position: absolute;

left: 15px;
    top: 14px;

color: #aaa;

pointer-events: none;

transition: 0.3s;

font-size: 14px;
}

.input-box input:focus + label,
.input-box input:not(:placeholder-shown) + label {
    top: -9px;
    left: 12px;

padding: 0 6px;

background: #24003a;

color: #ff4da6;

font-size: 11px;
}

/* =========================
   BUTTON
========================= */

button {
    width: 100%;

padding: 14px;

border: none;

border-radius: 10px;

cursor: pointer;

color: #fff;

font-weight: bold;

font-size: 15px;

background:
        linear-gradient(
            90deg,
            #ff0080,
            #7928ca,
            #ff0080
        );

background-size: 200% auto;

box-shadow:
        0 0 12px rgba(255,0,128,0.6);

transition: 0.3s;
}

button:hover {
    background-position: right center;

transform: translateY(-2px);

box-shadow:
        0 0 20px rgba(255,0,128,0.9),
        0 0 40px rgba(121,40,202,0.5);
}

/* =========================
   LINKS
========================= */

.links {
    text-align: center;

margin-top: 20px;
}

.links a {
    color: #ff4da6;

text-decoration: none;

font-size: 13px;

cursor: pointer;

transition: 0.3s;
}

.links a:hover {
    color: #fff;

text-shadow:
        0 0 10px #ff0080;
}

/* =========================
   MESSAGE
========================= */

.message {
    min-height: 22px;

margin-top: 15px;

text-align: center;

font-size: 13px;

font-weight: bold;

opacity: 0;
}

.message.show {
    opacity: 1;
}

.message.success {
    color: #00ffae;

text-shadow:
        0 0 10px rgba(0,255,174,0.8);
}

.message.error {
    color: #ff3860;

text-shadow:
        0 0 10px rgba(255,0,70,0.8);
}

/* =========================
   ERROR SHAKE
========================= */

.error-shake {
    animation: errorShake 0.45s ease;
}

@keyframes errorShake {

0% {
        transform: translateX(0);
    }

20% {
        transform: translateX(-10px);
    }

40% {
        transform: translateX(10px);
    }

60% {
        transform: translateX(-8px);
    }

80% {
        transform: translateX(8px);
    }

100% {
        transform: translateX(0);
    }
}

/* =========================
   ERROR INPUT
========================= */

.input-error {
    border-color: #ff1744 !important;

box-shadow:
        0 0 8px rgba(255,23,68,0.7),
        0 0 20px rgba(255,23,68,0.35) !important;
}

/* =========================
   RESPONSIVE
========================= */

@media(max-width:600px) {

.container {
        transform: scale(0.85);
    }

.bg-triangle {
        width: 500px;
        height: 500px;
    }
}

@media(max-width:420px) {

.container {
        transform: scale(0.75);
    }
}

const flipCard =
    document.getElementById("flipCard");

const loginForm =
    document.getElementById("loginForm");

const signupForm =
    document.getElementById("signupForm");

const showSignup =
    document.getElementById("showSignup");

const showLogin =
    document.getElementById("showLogin");

const loginMessage =
    document.getElementById("loginMessage");

const signupMessage =
    document.getElementById("signupMessage");

const username =
    document.getElementById("username");

const password =
    document.getElementById("password");

/* =========================
   SHOW SIGNUP
========================= */

showSignup.addEventListener("click", function(e) {

e.preventDefault();

flipCard.classList.add("flipped");

clearMessage(loginMessage);

});

/* =========================
   SHOW LOGIN
========================= */

showLogin.addEventListener("click", function(e) {

e.preventDefault();

flipCard.classList.remove("flipped");

clearMessage(signupMessage);

});

/* =========================
   LOGIN
========================= */

loginForm.addEventListener("submit", function(e) {

e.preventDefault();

clearMessage(loginMessage);

username.classList.remove("input-error");
    password.classList.remove("input-error");

const user =
        username.value.trim();

const pass =
        password.value.trim();

/*
       DEMO LOGIN

Username: admin
       Password: 123456
    */

if (
        user === "admin" &&
        pass === "123456"
    ) {

loginMessage.textContent =
            "✓ Login successful!";

loginMessage.className =
            "message success show";

}

else {

/* Add red input effect */

username.classList.add("input-error");
        password.classList.add("input-error");

/* Shake form */

const card =
            loginForm.closest(".form-card");

card.classList.remove("error-shake");

void card.offsetWidth;

card.classList.add("error-shake");

/* Error message */

loginMessage.textContent =
            "✕ Invalid username or password";

loginMessage.className =
            "message error show";

}

});

/* =========================
   SIGNUP
========================= */

signupForm.addEventListener("submit", function(e) {

e.preventDefault();

clearMessage(signupMessage);

const name =
        document.getElementById("signupName").value.trim();

const email =
        document.getElementById("signupEmail").value.trim();

const pass =
        document.getElementById("signupPassword").value.trim();

if (
        name === "" ||
        email === "" ||
        pass === ""
    ) {

signupMessage.textContent =
            "✕ Please fill all fields";

signupMessage.className =
            "message error show";

return;
    }

if (pass.length < 6) {

signupMessage.textContent =
            "✕ Password must be at least 6 characters";

signupMessage.className =
            "message error show";

return;
    }

signupMessage.textContent =
        "✓ Account created successfully!";

signupMessage.className =
        "message success show";

setTimeout(function() {

flipCard.classList.remove("flipped");

username.value = email;

password.value = "";

clearMessage(signupMessage);

}, 1500);

});

/* =========================
   CLEAR MESSAGE
========================= */

function clearMessage(element) {

element.textContent = "";

element.className =
        "message";

}

- Animated background Login UI

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