odoo18/addons/html_editor/static/src/main/hint.scss

.o-we-hint {
    position: relative;

    &:after {
        content: attr(placeholder);
        position: absolute;
        top: 0;
        left: 0;
        display: block;
        color: inherit;
        opacity: 0.4;
        pointer-events: none;
        text-align: inherit;
        width: 100%;
    }
}