.flash{
    overflow: hidden;
    position: relative;
    cursor: pointer;
}
.flash::before{
    content:'';
    position: absolute;
    top:0;
    left:-100%;
    width: 100%;
    height: 100%;
    background:linear-gradient(
        90deg,
        transparent,
        hsl(from var(--colour-white) h s l/.3),
        transparent
    );
    transition: .3s ease-in-out;
}
.flash:hover::before{
    left:100%;
}