CSS

make a box card responsive

@media screen and (min-width: 40em) {
    .cards {
        display: flex;
        flex-wrap: wrap;
        justify-content: space-between;
    }
 
    .card {
        flex: 0 1 calc(50% - 1em);
    }
}

responsive card css

/*I really love Bootstrap for these kind of things:*/

<div class="card-group">
  <div class="card">
    <img src="..." class="card-img-top" alt="...">
    <div class="card-body">
      <h5 class="card-title">Card title</h5>
      <p class="card-text">This is a wider card with supporting text below as a natural lead-in to additional content. This content is a little bit longer.</p>
    </div>
    <div class="card-footer">
      <small class="text-muted">Last updated 3 mins ago</small>
    </div>
  </div>
  <div class="card">
    <img src="..." class="card-img-top" alt="...">
    <div class="card-body">
      <h5 class="card-title">Card title</h5>
      <p class="card-text">This card has supporting text below as a natural lead-in to additional content.</p>
    </div>
    <div class="card-footer">
      <small class="text-muted">Last updated 3 mins ago</small>
    </div>
  </div>
  <div class="card">
    <img src="..." class="card-img-top" alt="...">
    <div class="card-body">
      <h5 class="card-title">Card title</h5>
      <p class="card-text">This is a wider card with supporting text below as a natural lead-in to additional content. This card has even longer content than the first to show that equal height action.</p>
    </div>
    <div class="card-footer">
      <small class="text-muted">Last updated 3 mins ago</small>
    </div>
  </div>
</div>

make a box card responsive

.cards {
   display: flex;
   flex-wrap: wrap;
   justify-content: space-between;
}