CODE EXAMPLE FOR CSS

css flex cards

@gray-darker:               #444444;
@gray-dark:                 #696969;
@gray:                      #999999;
@gray-light:                #cccccc;
@gray-lighter:              #ececec;
@gray-lightest:             lighten(@gray-lighter,4%);

*,
*::before,
*::after { 
  box-sizing: border-box;
}

html {
  background-color: #f0f0f0;
}

body {
  color: @gray;
  font-family: 'Roboto','Helvetica Neue', Helvetica, Arial, sans-serif;
  font-style: normal;
  font-weight: 400;
  letter-spacing: 0;
  padding: 1rem;
  text-rendering: optimizeLegibility;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  -moz-font-feature-settings: "liga" on;
}

img {
  height: auto;
  max-width: 100%;
  vertical-align: middle;
}

.btn {
  background-color: white;
  border: 1px solid @gray-light;
  //border-radius: 1rem;
  color: @gray-dark;
  padding: 0.5rem;
  text-transform: lowercase;
}

.btn--block {
  display: block;
  width: 100%;
}
 
.cards {
  display: flex;
  flex-wrap: wrap;
  list-style: none;
  margin: 0;
  padding: 0;
}

.cards__item {
  display: flex;
  padding: 1rem;
  @media(min-width: 40rem) {
    width: 50%;
  }
  @media(min-width: 56rem) {
    width: 33.3333%;
  }
}

.card {
  background-color: white;
  border-radius: 0.25rem;
  box-shadow: 0 20px 40px -14px rgba(0,0,0,0.25);
  display: flex;
  flex-direction: column;
  overflow: hidden;
  &:hover {
    .card__image {
      filter: contrast(100%);
    }
  }
}

.card__content {
  display: flex;
  flex: 1 1 auto;
  flex-direction: column;
  padding: 1rem;
}

.card__image {
  background-position: center center;
  background-repeat: no-repeat;
  background-size: cover;
  border-top-left-radius: 0.25rem;
  border-top-right-radius: 0.25rem;
  filter: contrast(70%);
  //filter: saturate(180%);
  overflow: hidden;
  position: relative;
  transition: filter 0.5s cubic-bezier(.43,.41,.22,.91);;
  &::before {
    content: "";
	  display: block;
    padding-top: 56.25%; // 16:9 aspect ratio
  }
  @media(min-width: 40rem) {
    &::before {
      padding-top: 66.6%; // 3:2 aspect ratio
    }
  }
}

.card__image--flowers {
  background-image: url(https://unsplash.it/800/600?image=82);
}

.card__image--river {
  background-image: url(https://unsplash.it/800/600?image=11);
}

.card__image--record {
  background-image: url(https://unsplash.it/800/600?image=39);
}

.card__image--fence {
  background-image: url(https://unsplash.it/800/600?image=59);
}

.card__title {
  color: @gray-dark;
  font-size: 1.25rem;
  font-weight: 300;
  letter-spacing: 2px;
  text-transform: uppercase;
}

.card__text {
  flex: 1 1 auto;
  font-size: 0.875rem;
  line-height: 1.5;
  margin-bottom: 1.25rem;
}




<ul class="cards">
  <li class="cards__item">
    <div class="card">
      <div class="card__image card__image--fence"></div>
      <div class="card__content">
        <div class="card__title">Flex</div>
        <p class="card__text">This is the shorthand for flex-grow, flex-shrink and flex-basis combined. The second and third parameters (flex-shrink and flex-basis) are optional. Default is 0 1 auto. </p>
        <button class="btn btn--block card__btn">Button</button>
      </div>
    </div>
  </li>
  <li class="cards__item">
    <div class="card">
      <div class="card__image card__image--river"></div>
      <div class="card__content">
        <div class="card__title">Flex Grow</div>
        <p class="card__text">This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.</p>
        <button class="btn btn--block card__btn">Button</button>
      </div>
    </div>
  </li>
  <li class="cards__item">
    <div class="card">
      <div class="card__image card__image--record"></div>
      <div class="card__content">
        <div class="card__title">Flex Shrink</div>
        <p class="card__text">This defines the ability for a flex item to shrink if necessary. Negative numbers are invalid.</p>
        <button class="btn btn--block card__btn">Button</button>
      </div>
    </div>
  </li>
  <li class="cards__item">
    <div class="card">
      <div class="card__image card__image--flowers"></div>
      <div class="card__content">
        <div class="card__title">Flex Basis</div>
        <p class="card__text">This defines the default size of an element before the remaining space is distributed. It can be a length (e.g. 20%, 5rem, etc.) or a keyword. The auto keyword means "look at my width or height property."</p>
        <button class="btn btn--block card__btn">Button</button>
      </div>
    </div>
  </li>
</ul>