.btn {
    position: relative;
}

.btn::before, .header::before {
    content: "";
    position: absolute;
    left: 0;
    right: 0;
    bottom: -10px;
    height: 30px;
    background-image:
       url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='600' height='30' viewBox='0 0 600 30'%3E%3Cpath d='M0 28 C 60 20, 120 36, 180 28 S 300 20, 360 28 S 480 36, 540 28 S 580 28, 600 28' fill='none' stroke='%238a93a6' stroke-width='2' stroke-linecap='round' opacity='0.65'/%3E%3C/svg%3E"),
        url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' width='600' height='30' viewBox='0 0 600 30'%3E%3Cg%3E%3Ccircle cx='40' cy='22' r='4.6' fill='%23ff3b3b'/%3E%3Ccircle cx='95' cy='34' r='4.6' fill='%23ffd23b'/%3E%3Ccircle cx='150' cy='25' r='4.6' fill='%2326e6a6'/%3E%3Ccircle cx='205' cy='30' r='4.6' fill='%233b82ff'/%3E%3Ccircle cx='260' cy='18' r='4.6' fill='%23ff3b3b'/%3E%3Ccircle cx='315' cy='28' r='4.6' fill='%23ffd23b'/%3E%3Ccircle cx='370' cy='25' r='4.6' fill='%2326e6a6'/%3E%3Ccircle cx='425' cy='38' r='4.6' fill='%233b82ff'/%3E%3Ccircle cx='480' cy='28' r='4.6' fill='%23ff3b3b'/%3E%3Ccircle cx='535' cy='34' r='4.6' fill='%23ffd23b'/%3E%3C/g%3E%3C/svg%3E");
    background-repeat: repeat-x, repeat-x;
    background-size: 600px 56px, 600px 56px;
    background-position: left bottom, left bottom;

    /* мерцание */
    filter: drop-shadow(0 0 6px rgba(255, 255, 255, .5));
    animation: lights-flicker 0.7s infinite ease-in-out;
}

@keyframes lights-flicker {

    0%,
    100% {
        filter: drop-shadow(0 0 4px rgba(255, 255, 255, .18));
    }

    40% {
        filter: grayscale(1);
    }

    75% {
        filter: drop-shadow(0 0 3px rgba(255, 255, 255, .14));
    }
}