```css
/* =========================
   BASIC RESET
========================= */

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

html {
    scroll-behavior: smooth;
}

body {
    min-height: 100vh;
    background: #fff5f7;
    color: #4a2630;
    font-family: "Poppins", sans-serif;
    overflow-x: hidden;
}


/* =========================
   OPENING SCREEN
========================= */

.hero {
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
    padding: 40px 20px;
    position: relative;
    z-index: 2;

    transition:
        opacity 0.8s ease,
        transform 0.8s ease;
}

.hero.opened {
    opacity: 0;
    transform: translateY(-60px);
    pointer-events: none;
}

.small-text {
    font-size: 14px;
    letter-spacing: 2px;
    text-transform: uppercase;
    color: #a66b78;
    margin-bottom: 10px;
}

h1 {
    font-family: "Great Vibes", cursive;
    font-size: clamp(70px, 12vw, 120px);
    font-weight: 400;
    color: #8e4053;
    margin-bottom: 35px;
}


/* =========================
   ENVELOPE
========================= */

.envelope-container {
    width: 300px;
    height: 210px;
    perspective: 1000px;
    margin-bottom: 30px;
}

.envelope {
    width: 100%;
    height: 100%;
    position: relative;
}

.envelope-back {
    position: absolute;
    inset: 0;
    background: #e9a8b6;
    border-radius: 4px;
    box-shadow: 0 20px 50px rgba(100, 30, 50, 0.18);
}

.envelope-front {
    position: absolute;
    bottom: 0;
    width: 0;
    height: 0;
    border-left: 150px solid transparent;
    border-right: 150px solid transparent;
    border-bottom: 115px solid #d98598;
    z-index: 4;
}

.flap {
    position: absolute;
    top: 0;
    left: 0;
    width: 0;
    height: 0;
    border-left: 150px solid transparent;
    border-right: 150px solid transparent;
    border-top: 115px solid #f0b8c4;
    z-index: 5;
    transform-origin: top center;
    transition: transform 0.8s ease;
}

.letter {
    position: absolute;
    width: 250px;
    height: 175px;
    background: white;
    left: 25px;
    top: 20px;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 45px;
    border-radius: 3px;
    z-index: 2;

    transition:
        transform 0.9s ease,
        z-index 0s 0.45s;
}

.envelope:hover .letter {
    transform: translateY(-10px);
}


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

.open-button {
    border: none;
    padding: 15px 28px;
    border-radius: 50px;
    background: #8e4053;
    color: white;
    font-family: "Poppins", sans-serif;
    font-size: 15px;
    cursor: pointer;

    box-shadow: 0 10px 25px rgba(142, 64, 83, 0.25);

    transition:
        transform 0.25s ease,
        box-shadow 0.25s ease;
}

.open-button:hover {
    transform: translateY(-3px);
    box-shadow: 0 15px 30px rgba(142, 64, 83, 0.3);
}

.hint {
    color: #9c6873;
    font-size: 13px;
    margin-bottom: 18px;
}


/* =========================
   LETTER
========================= */

.letter-section {
    min-height: 100vh;
    padding: 80px 20px 100px;

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

    opacity: 0;
    transform: translateY(40px);

    transition:
        opacity 1s ease,
        transform 1s ease;
}

.letter-section.visible {
    opacity: 1;
    transform: translateY(0);
}

.letter-card {
    width: min(750px, 100%);
    background: rgba(255, 255, 255, 0.9);

    padding: clamp(35px, 7vw, 70px);

    border-radius: 12px;

    box-shadow:
        0 30px 80px rgba(100, 30, 50, 0.12);

    position: relative;
}

.date {
    text-align: center;
    color: #b07884;
    font-size: 12px;
    letter-spacing: 2px;
    text-transform: uppercase;
    margin-bottom: 35px;
}

h2 {
    font-family: "Great Vibes", cursive;
    font-size: 65px;
    font-weight: 400;
    color: #8e4053;
    margin-bottom: 30px;
}

.letter-text {
    font-family: "DM Serif Display", serif;
    font-size: 19px;
    line-height: 1.9;
    color: #57353e;
}

.letter-text p {
    margin-bottom: 25px;
}

.letter-text em {
    color: #8e4053;
}

.big-love {
    text-align: center;
    font-family: "Great Vibes", cursive;
    font-size: 65px;
    color: #8e4053;
    margin: 45px 0 !important;
}

.divider {
    text-align: center;
    color: #c77a8b;
    font-size: 28px;
    margin: 35px 0;
}

.ending {
    text-align: center;
    font-family: "DM Serif Display", serif;
    font-size: 21px;
    line-height: 1.7;
}

.signature {
    text-align: right;
    margin-top: 35px;
    font-family: "Great Vibes", cursive;
    font-size: 42px;
    color: #8e4053;
}


/* =========================
   FLOATING HEARTS
========================= */

.hearts {
    position: fixed;
    inset: 0;
    overflow: hidden;
    pointer-events: none;
    z-index: 1;
}

.hearts span {
    position: absolute;
    bottom: -50px;
    color: #d98598;
    opacity: 0.35;
    font-size: 25px;

    animation: floatHeart linear infinite;
}

.hearts span:nth-child(1) {
    left: 10%;
    animation-duration: 12s;
    animation-delay: 0s;
}

.hearts span:nth-child(2) {
    left: 25%;
    animation-duration: 15s;
    animation-delay: 3s;
}

.hearts span:nth-child(3) {
    left: 40%;
    animation-duration: 11s;
    animation-delay: 1s;
}

.hearts span:nth-child(4) {
    left: 55%;
    animation-duration: 14s;
    animation-delay: 5s;
}

.hearts span:nth-child(5) {
    left: 70%;
    animation-duration: 13s;
    animation-delay: 2s;
}

.hearts span:nth-child(6) {
    left: 82%;
    animation-duration: 16s;
    animation-delay: 4s;
}

.hearts span:nth-child(7) {
    left: 92%;
    animation-duration: 12s;
    animation-delay: 7s;
}

.hearts span:nth-child(8) {
    left: 3%;
    animation-duration: 17s;
    animation-delay: 6s;
}

@keyframes floatHeart {

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

    15% {
        opacity: 0.35;
    }

    100% {
        transform: translateY(-110vh) rotate(360deg);
        opacity: 0;
    }

}


/* =========================
   FOOTER
========================= */

footer {
    text-align: center;
    padding: 30px;
    color: #a66b78;
    font-size: 12px;
}


/* =========================
   MOBILE
========================= */

@media (max-width: 600px) {

    h1 {
        font-size: 75px;
    }

    .envelope-container {
        transform: scale(0.85);
        margin-top: -10px;
        margin-bottom: 10px;
    }

    .letter-card {
        padding: 35px 25px;
    }

    h2 {
        font-size: 52px;
    }

    .letter-text {
        font-size: 17px;
        line-height: 1.8;
    }

    .big-love {
        font-size: 52px;
    }

    .signature {
        font-size: 38px;
    }

}
```