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CSS

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HTML 

              
                <main>
  <h1>Text Contrast Issues That Cannot Be Detected with Automated Tools</h1>

  <p>
    These are based on real-world examples that we have encountered in audits, though this is simplified to show the salient bits.
  </p>

  <h2>Text on a Background Gradient</h2>

  <p>
    Pretend this is one "slide" from a carousel, which is where I see this crop up most frequently.
  </p>

  <div>
    <p>
      This is some sort of marketing text, usually justifying some sort of call to action following.
      <a href="#">Call to Action!</a>
    </p>
  </div>

  <h2>Text on a Background Texture</h2>

  <p>
    This is a CSS background texture. While the colors are at least embedded in the code, where they may fall below the text is impossible to determine
  </p>

  <div>
    <p>
      This is some sort of marketing text, usually justifying some sort of call to action following.
      <a href="#">Call to Action!</a>
    </p>
  </div>

  <h2>Text on a Background Image</h2>

  <p>
    In responsive layouts in particular it can be very hard to identify where an image will sit below the text.
  </p>

  <div>
    <p>
      This is some sort of marketing text, usually justifying some sort of call to action following.
      <a href="#">Call to Action!</a>
    </p>
  </div>

  <h2>Text on an Animated Background Image</h2>

  <p>
    This is a CSS animation, but an animated GIF can be just as hard to map to the text with an automated tool.
  </p>

  <div>
    <p>
      This is some sort of marketing text, usually justifying some sort of call to action following.
      <a href="#">Call to Action!</a>
    </p>
  </div>

  <h2>Text on an Animated Background Texture</h2>

  <p>
    Same as above, but with CSS only.
  </p>

  <div>
    <p>
      This is some sort of marketing text, usually justifying some sort of call to action following.
      <a href="#">Call to Action!</a>
    </p>
  </div>

  <h2>Text Absolutely Positioned over a Box</h2>

  <p>
    In this case the text is not in a container. Oh. Nope. This one was flagged. Good job, tool.
  </p>

  <div>
  </div>
  <p>
    This is some sort of marketing text, usually justifying some sort of call to action following.
    <a href="#">Call to Action!</a>
  </p>
  


  <h2>Text with A Contrasty "Outline"</h2>

  <p>
    In this example, the link would trigger a false positive. The text is the same color as the background, but with a border-like style to make it stand out more.
  </p>

  <div>
    <p>
      This is some sort of marketing text, usually justifying some sort of call to action following.
      <a href="#">Call to Action!</a>
    </p>
  </div>
</main>
!

CSS 

              
                @import url(https://fonts.googleapis.com/css?family=Lato:400,700);
body {
  background-color: #fff;
  font-size: 100%;
  color: #333;
  font-family: Lato, Arial, sans-serif;
  padding: 0;
  margin: 0;
}

main {
  display: block;
  box-sizing: border-box;
  margin: 1em .5em;
  padding: 0 0;
  color: #000;
  line-height: 1.5;
}

input {
  font-size: 100%;
  color: #333;
  font-family: Lato, Arial, sans-serif;
  border: .08em solid #333;
  padding: .25em .25em;
}

button {
  font-size: 2em;
  border: .1em solid #000;
  background-color: #fff;
  color: #000;
  border-radius: 50%;
  width: 1.5em;
  height: 1.5em;
  margin-left: 1em;
}

button:hover,
button:focus {
  background-color: #000;
  color: #fff;
}

main > h2 {
  clear: both;
}

main > div {
  display: block;
  box-sizing: border-box;
  margin: 1em auto;
  padding: 2.5em;
  color: #000;
  background-color: rgba(204, 204, 204, .2);
  border: .07em solid rgba(0, 0, 0, .8);
  border-radius: .25em;
  clear: both;
}

main div:nth-of-type(1) {
  background-image: linear-gradient(to top, #30cfd0 0%, #330867 70%);
}

main div:nth-of-type(2) {
  background: linear-gradient(45deg, #dca 12%, transparent 0, transparent 88%, #dca 0), linear-gradient(135deg, transparent 37%, #a85 0, #a85 63%, transparent 0), linear-gradient(45deg, transparent 37%, #dca 0, #dca 63%, transparent 0) #753;
  background-size: 25px 25px;
}

main div:nth-of-type(3) {
  background-image: url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/2729/shape-outside-2.jpg);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: 0 80%;
}

main div:nth-of-type(4) {
  background-color: rgb(247, 231, 206);
  background: linear-gradient(to right, rgba(229, 196, 146, .5) 0, rgba(247, 231, 206, .25) 25%, rgba(247, 231, 206, .25) 75%, rgba(229, 196, 146, .5)), url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/2729/drops1.png) 100% 0 repeat, url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/2729/drops2.png) 50% 50% repeat, url(https://s3-us-west-2.amazonaws.com/s.cdpn.io/2729/drops3.png) 0 0 repeat, linear-gradient(to right, rgb(229, 196, 146) 0, rgb(247, 231, 206) 25%, rgb(247, 231, 206) 75%, rgb(229, 196, 146));
  background-size: cover, 350px, 250px, 150px, cover;
  background-attachment: fixed, fixed, fixed, fixed;
  -webkit-animation: EFFERVESCE 3s linear infinite;
  animation: EFFERVESCE 8s linear infinite;
}

main div:nth-of-type(5) {
  background: linear-gradient(135deg, #ECEDDC 25%, transparent 25%), linear-gradient(225deg, #ECEDDC 25%, transparent 25%), linear-gradient(315deg, #ECEDDC 25%, transparent 25%), linear-gradient(45deg, #ECEDDC 25%, transparent 25%);
  background-size: 100px 100px;
  background-color: #c00;
  animation: ZIGGYZAGGY 16s linear infinite;
}

main div:nth-of-type(6) {
  background-color: #333;
  height: 15em;
}

main div:nth-of-type(6) + p {
  box-sizing: border-box;
  padding: 2.5em;
  position: absolute;
  margin-top: -15.5em;
  margin-right: 1em;
}

main > div a,
main div:nth-of-type(6) + p a {
  display: block;
  box-sizing: border-box;
  width: 10em;
  margin: 1em auto 0 auto;
  padding: .5em 0;
  text-align: center;
  border: .07em solid rgba(255, 255, 255, .8);
  background-color: #33c;
  border-radius: .5em;
  text-decoration: none;
  color: #fff;
  font-weight: bold;
  font-size: .8em;
}

main div:nth-of-type(7) a {
  font-size: 200%;
  font-weight: bold;
  color: #33c;
  text-shadow: 0.02em 0.02em 0 #ff0, -0.02em -0.02em 0 #ff0, -0.02em 0.02em 0 #ff0, 0.02em -0.02em 0 #ff0, 0 0.02em 0 #ff0, 0 -0.02em 0 #ff0, -0.02em 0 0 #ff0, 0.02em 0 0 #ff0;
}

main > div a:hover,
main > div a:focus,
main div:nth-of-type(6) + p a:hover,
main div:nth-of-type(6) + p a:focus {
  background-color: #090;
  box-shadow: 0 0 1em #000;
}

@media all and (min-width: 36em) {
  main {
    margin: 1em 2em;
  }
  main > div,
  main div:nth-of-type(6) + p {
    padding: 2.5em 11em 2.5em 4em;
    font-size: calc(10px + 2vw);
  }
  form > div::after {
    content: "";
    display: block;
    clear: both;
  }
}

@-webkit-keyframes EFFERVESCE {
  from {
    background-position: 0 0, 0 0, 0 0, 0 0, 0 0;
  }
  to {
    background-position: 0 0, 0 -700px, 0 -500px, 0 -300px, 0 0;
  }
}

@keyframes EFFERVESCE {
  from {
    background-position: 0 0, 0 0, 0 0, 0 0, 0 0;
  }
  to {
    background-position: 0 0, 0 -700px, 0 -500px, 0 -300px, 0 0;
  }
}

@keyframes ZIGGYZAGGY {
  from {
    background-position: 25% 0, 25% 0, 0 0, 0 0;
  }
  to {
    background-position: 125% 0, 125% 0, 100% 0, 100% 0;
  }
}
!

JS 

              
                /* Getting around CodePen's inability to allow me to set a lang on the <html> element */

var h = document.querySelector("html"); 
h.setAttribute("lang", "en");
!
999px

Console