I have been challenged by my mentor to add vertical, slide animation to my nav bar in my sandbox-type project USING ONLY CSS & HTML. I have tried countless things at this point and need some help -- at the very least some direction. I have tried: several animation names, transform and translate3d, shifting elements on their axis & rotating, etc.
.collapsible-menu label {
    font-family: 'Chiller';
    font-size: 70px;
    display: block;
    cursor: pointer;
    background: url(C:/Users/audre/workspace/basic/assets/menu.png) no-repeat left center;
    background-size: 15%;
    padding: 10px 0px 0px 50px;
}
input#menu {
    display: none;
}
.menu-content {
    max-height: 0;
    font-family: 'Oswald', sans-serif; 
    padding: 0 0 0 50px;
    animation: 3s linear running slidein;
}
/* Toggle Effect */
input:checked +label {
    background-image: url(C:/Users/audre/workspace/basic/assets/cancel.png);  
}
input:checked ~ .menu-content {
    max-height: 100%;
}
/*Icons made by https://www.flaticon.com/authors/cole-bemis from www.flaticon.com/ */
.collapsible-menu {
    background-color: rgb(255, 255, 255);
    padding: 0px 30px;
    width: 200px;
    border-bottom: 3px solid #490000;
    box-shadow: 1px 2px 3px rgba(0,0,0,0.2);
    z-index: 5;
}
.collapsible-menu ul {
    list-style-type: none;
    padding: 0;
}
.collapsible-menu a {
    display: block;
    padding: 10px;
    text-decoration: none;
} <div class="collapsible-menu">
        <input type="checkbox" id="menu">
        <label for="menu">Menu</label>
        <nav class="menu-content">
            <ul>
                <li><a class="active" href="#home"></a>Home</li>
                <li><a href="./about.html" target="_blank"></a>About Audrey</li>
                <li><a href="#shop"></a>Shop</li>
                <li><a href="#contact"></a>Contact</li>
            </ul>
        </nav>
    </div>Any help or tips are greatly appreciated! I'm a little bit at a loss here.
What I have tried (cliffnotes version):
- @keyframes definition for a "slidein" rule
- transition declaration with various adjustments/toying around with
- animation declaration with various adjustments/toying around with
EDITS/UPDATES:
- Edited code to be the minimal reproducible example
- Cleaned up code of unnecessary declarations
- Added summary of what I have tried thus far
 
    