一键变灰

// css
html {
    filter：grayscale(100%)
}

首屏变灰

// css
html {
    position: relative;
    width: 100%;
    height: 100%;
}
html::before {
    content: "";
    position: absolute;
    backdrop-filter: grayscale(100%);
    pointer-events: none;
    inset: 0;
    z-index: 9999;
}

全部置灰

// css
html {
    position: relative;
    width: 100%;
    height: 100%;
}
html::before {
    content: "";
    position: fixed;
    backdrop-filter: grayscale(100%);
    pointer-events: none;
    inset: 0;
    z-index: 9999;
}