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HTML 

              
                <!-- in a wrapping container include a div.card used to design the card  -->
<div class="container">
  <div class="card">
    <!-- in the card display a header and a div each for each option  -->

    <h2>As you evaluate your site, consider if your content is:</h2>

    <div class="card__option">
      <!-- for each option include a div in which to include the description and a span used to display a + sign, to remark the toggle-able nature of the option -->
      <div class="card__option--description">
        <!-- for the description, include a term with dt element and an illustration of said term with a dd element  -->
        <dl>
          <dt>
            perceivable
          </dt>
          <dd>
            Can people know that your cotent exists with more than one sense?
          </dd>
        </dl>
      </div>
      <span class="card__option--toggle">+</span>
    </div>

    <div class="card__option">
      <div class="card__option--description">
        <dl>
          <dt>
            understandable
          </dt>
          <dd>
            Can users take in and comprehend the information in a variety of ways?
          </dd>
        </dl>
      </div>
      <span class="card__option--toggle">+</span>
    </div>

    <div class="card__option">
      <div class="card__option--description">
        <dl>
          <dt>
            operable
          </dt>
          <dd>
            Can users interact with and navigate your website?
          </dd>
        </dl>
      </div>
      <span class="card__option--toggle">+</span>
    </div>

    <div class="card__option">
      <div class="card__option--description">
        <dl>
          <dt>
            robust
          </dt>
          <dd>
            Can people access your site using various browsers, operating systems, and assistive tools?
          </dd>
        </dl>
      </div>
      <span class="card__option--toggle">+</span>
    </div>

  </div><!-- close div.card -->

</div><!-- close div.container -->
!

CSS 

              
                @import url('https://fonts.googleapis.com/css?family=Open+Sans');

font-main = 'Open Sans', sans-serif
black = #252525
green = #84BC45

*
    box-sizing border-box
    margin 0
    padding 0
  
body
    min-height 100vh
    width 100%
    font-family font-main
    color black
    background green

div.container 
    display flex
    justify-content center 
    align-items center 
    min-height 100vh
    padding 1rem 2rem

    div.card 
        background #fff
        height 100vh 
        width (@height * 0.8)
        filter drop-shadow(0 1px 5px rgba(black, 0.3))
        // transition the card into view
        animation slideIn 0.3s 0.3s ease-out both
        // display the contents of the div.card in a single column layout
        display flex 
        flex-direction column

        h2, div.card__option
            // set the header and div(s) element to each cover a portion of the card 
            flex-grow 1
            padding 2rem

        h2 
            font-weight 300

        div.card__option
            // lay the div containing the text and the span containing the plus + sign in a row
            display flex 
            align-items center
            justify-content space-between
            cursor pointer
            // set transition for the change in background, prompted in the JS script
            transition all 0.3s ease

            span.card__option--toggle 
                font-size 2rem
                font-weight bold

            div.card__option--description
                dl
                    // in a dl element, dt and dd element are already displayed vertically one after the other
                    // no need to specify further layout properties (like flex)

                    dt
                        font-weight bold 
                        font-size 1.15rem
                        letter-spacing 0.05rem
                        text-transform uppercase
                        margin 0.2rem 0

                    dd 
                        font-size 1.1rem
                        // by default hide the element
                        opacity 0
                        visibility hidden
                        // set height to 0 to allow for the dt element to be centered horizontally in the parent div
                        // otherwise, the element occupies space, even if not displayed on screen
                        height 0

                        &.active
                            // define a class which transitions the dd element into view (through JS instructions)
                            opacity 1
                            visibility visible
                            height 100%
                            // set transition only to the class to prevent the transition from occurring evenly as the class is added and removed
                            // this way the transition is set only when the class is added
                            // additionally, transition opacity with a delay, giving the time to the browser to accommodate the space necessary for the dd element
                            transition visibility 0.3s ease, opacity 0.3s 0.2s ease

// define the animation for the div.card element
@keyframes slideIn 
    0% 
        opacity 0
        transform translateX(-20%)
    100% 
        opacity 1
        transform translateX(0)
!

JS 

              
                // target all div containing the options
const cardOptions = document.querySelectorAll(".container .card .card__option");
// target all dd elements
const cardOptionsDescriptions = document.querySelectorAll(".container .card .card__option dl dd");
// target all span elements
const cardOptionsSpans = document.querySelectorAll(".container .card .card__option span");

// listen for a click event on each div containing each option, at which point call a function which displays the description of the clicked option 
cardOptions.forEach(cardOption => {
    cardOption.addEventListener("click", handleClick);
});

// create a function which takes as argument the (click) event and calls a function to display the correct option
function handleClick(e) {
    // pass as argument the HTML node actually clicked (this may be the div.card__option, but also one of its nested items, such as the dt, or span elements nested within)
    displayOption(e.target);
}

function displayOption(targetClick) {
    // store in a variable the clicked element, and update it in a loop in order to find the parent div.card__option
    let target = targetClick;

    // until the target is not the parent div.card__option (technically until the element contains a class of .card__option)
    while(!target.classList.contains("card__option")) {
        // update the element to go up a node
        target = target.parentNode;
    }

    // target the description element of the clicked div.card__option
    let descriptionTarget = target.querySelector("dd");

    // target the span element of the clicked div.card__option
    let spanTarget = target.querySelector("span");

    // if the dd element already has a class of active, remove it 
    // additionally reset the background of the parent container and add the plus sign + back
    if(descriptionTarget.classList.contains("active")) {
        target.style.background = "none";
        descriptionTarget.classList.remove("active");
        spanTarget.textContent = "+";
        
    }
    // otherwise call a function to remove the class of active from all options and add it exclusively to the only div.card__option 
    // additionally, style the parent container by changing its background and remove the plus sign from the nested span
    else {
        removeOptions();
        target.style.background = "rgba(0,200,0,0.2)";
        descriptionTarget.classList.add("active");
        spanTarget.textContent = " ";
    }
}

// create a function which removes the class of active from all dd elements 
// additionally reset the background of the parent container and restore the plus sign + on all span elements
function removeOptions() {
    cardOptions.forEach(cardOption => cardOption.style.background = "none");
    cardOptionsSpans.forEach(cardOptionsSpan => cardOptionsSpan.textContent = "+");
    cardOptionsDescriptions.forEach(cardOptionsDescription => cardOptionsDescription.classList.remove("active"));
}
!
999px

Console