How to reproduce this rollover effect

Question

This website 's nav bar has a nice, smooth rollover effect that I like. As far as I can tell, it's being done in JavaScript - can such a thing be easily emulated with CSS, and if so, how would I apply it to my own website's navigation?

I've looked for CSS examples, but nothing I've found quite comes close to the cascade-like effect that this website's menu has, and adapting what I have found to my (Wordpress) site just breaks my navigation.

The slow slide back when the site's sub-menu item loses focus would also be a massive bonus. If the effects can't be emulated to the same degree of smoothness in CSS, I'm happy to be directed to a JavaScript solution/library that I won't have to pay for.

Here's the markup for my site's navigation:

<nav class="main-navigation">
  <ul id="primary-menu" class="primary-menu menu">
    <li class="menu-item"><a href="http://localhost/wordpress">Home</a></li>
    <li class="menu-item"><a href="http://localhost/wordpress/about">About</a>
      <ul class="sub-menu">
        <li class="menu-item"><a href="http://localhost/wordpress/about/history/">History</a></li>
        <li class="menu-item"><a href="http://localhost/wordpress/about/people">People</a></li>
      </ul>
    </li>
    <li class="menu-item"><a href="http://localhost/wordpress/services/">Services</a></li>
    <li class="menu-item"><a href="http://localhost/wordpress/what-we-do">What We Do</a></li>
    <li class="menu-item"><a href="http://localhost/wordpress/the-library/">The Library</a>
      <ul class="sub-menu">
        <li class="menu-item"><a href="http://localhost/wordpress/the-library/gallery/">Gallery</a></li>
        <li id="menu-item" class="menu-item"><a href="http://localhost/wordpress/audio-and-video/">Audio and Video</a></li>
      </ul>
    </li>
    <li class="menu-item"><a href="http://localhost/wordpress/donate/">Donate</a></li>
    <li class="menu-item"><a href="http://localhost/wordpress/contact/">Contact</a></li>
  </ul>
</nav>

And here is most of the CSS relating to it:

.primary-menu,
.primary-menu ul {
    margin: 0;
    padding: 0;
    list-style: none;
}

.primary-menu {
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;
    -ms-flex-wrap: wrap;
    flex-wrap: wrap;
    margin-left: calc(var(--menuItemsSpacing) / -2);
    margin-right: calc(var(--menuItemsSpacing) / -2);
}

.primary-menu a {
    display: -webkit-box;
    display: -ms-flexbox;
    display: flex;
    -webkit-box-align: center;
    -ms-flex-align: center;
    align-items: center;
    text-align: initial;
    overflow: none;
    color: #000;
}

.primary-menu .current-menu-item a {
    color:#FFF;
    background-color: #09153F;
}

.primary-menu>li {
    font-size: 18px;
    font-family: 'Open Sans', sans-serif; 
    font-weight: 400;
    position:relative;
    margin-bottom: 20px;
}

.primary-menu>li a {
    padding: 10px 25px 10px 25px;
    -webkit-transition-duration: 0.3s;
    transition-duration: 0.3s;
    -webkit-transition-property: color, background-color;
    transition-property: color, background-color; 
}

.primary-menu>li:hover a,
.primary-menu>li:focus a,
.primary-menu>li:active a {
    background-color: #09153F;
    color: #FFF;

}

.primary-menu .sub-menu {
    position: absolute;
    top: 100%;
    margin-top: var(--dropdownTopOffset);
    min-width: 100%;
    width:120%;
    display:block;
    border-radius: 2px;
    background: #09153F;
}

.primary-menu .sub-menu li {
    font-size: 15px;
    font-weight: 400;
    text-transform: initial;
    line-height: 1.6;
}

.primary-menu .sub-menu li a {
    transition: padding 0.2s ease-out;
}

.primary-menu .sub-menu li a:hover {
    padding-left:30px;
    color: var(--menuHoverColor);
}

Answer 1

Look into CSS transitions.

Here is an example. ul.topLevel li { transition:all .6s ease-out } ul.topLevel li:hover { transition:all .2s ease-out }

Edit:

As per the [CodePen](https://codepen.io/toddsynan/pen/pJuyx) all that is needed is transitions.

.nav2 li ul {
    position: absolute;
    left: 0;
    top: 36px;
    z-index: 1;
    visibility: hidden;
    opacity: 0;
    filter: alpha(opacity=0);
    -webkit-transition: 200ms ease;
    -moz-transition: 200ms ease;
    -o-transition: 200ms ease;
    transition: 200ms ease;
}