.menu-ngang {
    display: flex;
    list-style: none;
    background: blue;
}

.menu-ngang li {
    position: relative;
}

.menu-ngang a {
    color: white;
    padding: 10px 15px;
    display: block;
    text-decoration: none;
}

.menu-ngang a:hover {
    background: darkblue;
}

/* submenu */
.menu-ngang ul {
    display: none;
    position: absolute;
    top: 100%;
    left: 0;
    background: blue;
    list-style: none;
}

/* cấp 3 */
.menu-ngang ul ul {
    left: 100%;
    top: 0;
}

.menu-ngang li:hover > ul {
    display: block;
}