JSFiddle - React, Tailwind, and code Playground

HTML

<div class="b-main-menu">
                        <ul class="b-main-menu__list">
                            <li class="b-main-menu__item">
                                <h2><a href="" class="b-main-menu__link">Подгузники</a></h2>
                            </li>
                            <li class="b-main-menu__item">
                                <h2><a href="" class="b-main-menu__link">Уход за малышом</a></h2>
                                <div class="b-sub-main-menu">
                                    <ul class="b-sub-main-menu__list b-sub-main-menu__list_left">
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Бутылочки, соски, пустышки</a></li>
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Стульчики для кормления</a></li>
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Посуда</a></li>
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Электроника</a></li>
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Аксессуары для кормления</a></li>
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Бутылочки, соски, пустышки</a></li>
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Стульчики для кормления</a></li>
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Посуда</a></li>
                                        <li class="b-sub-main-menu__item"><a class="b-sub-main-menu__link" href="#">Электроника</a></li>
                                    </ul>
                                    
                                </div>
     ...

CSS

.b-main-menu{
    margin:5px 0 0 4px;
    position:relative;
}
    .b-main-menu__list{
        -moz-border-radius:3px;
        -webkit-border-radius:3px;
        border-radius:3px;
        height:40px;
        margin:0 0 25px;
        padding:0 174px 0 10px;
    }
    .b-main-menu__item{
        background:#9bcdea;
        border-right:3px solid #fff;
        display:inline-block;
        height:40px;
        position: relative;    
    }
    .b-main-menu__item:hover{
        margin-right:-10px;
        padding-right:10px;
        margin-left:-10px;
        padding-left:10px;
        height:50px;
        background:#b3daf1;
    }
    .b-main-menu__item:last-child{
        border:none;
    }
    .b-main-menu__hover{
        height:51px;
        left:-3px;
        position:absolute;
        top:-3px;
        outline:1px solid blue;
    }
    .b-main-menu__link{
        display:inline-block;
        color:#555;
        height:20px;
        font:14px Arial,sans-serif;
        padding:11px 8px 11px 8px;
        text-decoration:none;
    }
    .b-main-menu__link:hover{
        text-decoration:underline;
    }
    
    .b-sub-main-menu {
        position: absolute; /*--Important--*/
        top: 44px; left:-5px;
        z-index: 99999;
        background:#d2e2eb;
        display: none; /*--Hidden for those with js turned off--*/
    }
    .b-sub-main-menu__list{
        display:inline-block;
        padding:0 15px 0 15px;
        vertical-align:top;
    }
            .b-sub-main-menu__list_left{
                background:#b3daf1;
                border-right:1px solid #d2e2eb;
                width:235px;
            }
                .b-sub-main-menu__list_left .b-sub-main-menu__item{
                    border-bottom:1px solid #cce0ec;
                    padding:10px 0;
                }
                .b-sub-main-menu__list_left .b-sub-main-menu__item:last-child{
                    border-bottom:none;
                }
                .b-sub-main-menu__list_left...

JavaScript

$('.b-main-menu__item').hover(function(){$(this).children('.b-sub-main-menu').stop(true, true).fadeToggle(300);});