I'm using the theme Illdy, and the hoverable dropdown menus on the top navbar are not dropping down in IE or Edge. Firefox and Chrome appear to work. I've tried adding a variety of these to the additional CSS:

#header .top-header .header-navigation ul li:hover > ul.sub-menu {
visibility: visible;
}

.header-navigation .menu > li:hover > ul {
opacity: 1;
transform: translate3d(0, 0px, 0);
-webkit-transform: translate3d(0, 0px, 0);
-moz-transform: translate3d(0, 0px, 0);
visibility: visible;
}
.header-navigation .menu-item>li>ul li:hover>a {
opacity: 1;
}

to no avail.

The theme's original CSS pertaining to the top navbar menu is:

#header .top-header .header-logo {
    display: block;
    font-size: 60px;
    color: #fff;
    -webkit-transition: all 0.3s;
    -moz-transition: all 0.3s;
    -o-transition: all 0.3s;
    transition: all 0.3s;
    line-height: 75px;
    font-weight: 700;
}

#header .top-header .header-logo:hover,
#header .top-header .header-logo:focus {
    color: #f1d204;
    text-decoration: none;
}

#header .top-header .header-navigation {
    float: right;
}

#header .top-header .header-navigation ul {
    width: 100%;
    margin: 0;
    padding: 0;
    list-style-type: none;
}

#header .top-header .header-navigation > ul {
    margin-top: 20px;
}

#header .top-header .header-navigation ul li {
    margin-left: 40px;
    line-height: 40px;
    font-weight: 700;
    font-size: 14px;
    color: #fff;
    position: relative;
    float: left;
    font-family: "Lato";
}

#header .top-header .header-navigation ul li:first-child {
    margin-left: 0;
}

#header .top-header .header-navigation ul li.menu-item-has-children a {
    padding-right: 16px;
    position: relative;
    margin-bottom: 20px;
    display: inline-block;
}

#header .top-header .header-navigation ul li.menu-item-has-children a:after {
    content: "\f107";
    font-family: "FontAwesome";
    font-size: 16px;
    position: absolute;
    top: 0;
    right: 0;
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu {
    width: 200px;
    margin: -5px 0 0;
    padding: 0;
    position: absolute;
    left: 50%;
    display: none;
    z-index: 10;
    background-color: #fff;
    -webkit-box-shadow: 0px 0px 20px 2px rgba(0, 0, 0, 0.32);
    -moz-box-shadow: 0px 0px 20px 2px rgba(0, 0, 0, 0.32);
    box-shadow: 0px 0px 20px 2px rgba(0, 0, 0, 0.32);

    transform: translateX(-50%);

}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu:before {
    display: block;
    content: "";
    position: absolute;
    top: -5px;
    left: 50%;
    margin-left: -3px;
    border-left: 5px solid transparent;
    border-right: 5px solid transparent;
    border-bottom: 5px solid #fff;
    width: 0;
    height: 0;
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu .sub-menu {
    transform: translateX(0);
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu .sub-menu:before {
    display: none;
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu li {
    width: 100%;
    margin: 0;
    padding: 0;
    background-color: #fff;
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu li:first-child {
    padding-top: 10px;
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu li:last-child {
    padding-bottom: 10px;
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu li a {
    width: 100%;
    margin: 0;
    padding: 0 10px;
    line-height: 40px;
    font-weight: 400;
    font-size: 14px;
    color: #8c9597;
    text-transform: none;
    display: block;
    font-family: "Lato";
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu li:hover > a,
#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu li:focus-within > a {
    color: #f1d204;
    border-left: 3px solid #f1d204;
    background-color: #f8f8f8;
}

#header .top-header .header-navigation ul li.menu-item-has-children .sub-menu li a:after {
    display: none;
}

#header .top-header .header-navigation ul li a {
    color: #fff;
    font-size: 16px;
    line-height: 26px;
}

#header .top-header .header-navigation ul li:hover a,
#header .top-header .header-navigation ul li:focus-within a {
    color: #ffde00;
    text-decoration: none;
}