MediaWiki:Skeleton.css

.tabber__panel:not(.smw-loaded) {
    position: relative;
    margin-top: 5px;
    min-height: 200px;
    pointer-events: none;
    overflow: hidden;
}

.tabber__panel:not(.smw-loaded) > * {
    opacity: 0;
    visibility: hidden;
}

.tabber__panel:not(.smw-loaded)::before {
    content: '';
    position: absolute;
    inset: 0;
    z-index: 10;
    background-color: #e0e0e0;
    border-radius: 8px;
}

.tabber__panel:not(.smw-loaded)::after {
    content: '';
    position: absolute;
    inset: 0;
    z-index: 11;
    background: linear-gradient(
        90deg,
        transparent 0%,
        rgba(255, 255, 255, 0.65) 45%,
        rgba(255, 255, 255, 0.85) 50%,
        rgba(255, 255, 255, 0.65) 55%,
        transparent 100%
    );
    animation: smw-shimmer 1.6s infinite ease-in-out;
    transform: translateX(-100%);
}

@keyframes smw-shimmer {
    0%   { transform: translateX(-100%); }
    100% { transform: translateX(100%); }
}

.tabber__panel.smw-loaded > * {
    animation: smw-fadein 0.35s ease forwards;
}

@keyframes smw-fadein {
    from { opacity: 0; }
    to   { opacity: 1; }
}