CSS

.alert {
  background-color:yellow;
  background-image:-webkit-repeating-linear-gradient(45deg, yellow 15px, yellow 35px, black 30px, black 60px);
  background-image:-moz-repeating-linear-gradient(45deg, yellow 15px, yellow 35px, black 30px, black 60px);
  background-image:-ms-repeating-linear-gradient(45deg, yellow 15px, yellow 35px, black 30px, black 60px);
  background-image:-o-repeating-linear-gradient(45deg, yellow 15px, yellow 35px, black 30px, black 60px);
  background-image:repeating-linear-gradient(45deg, yellow 15px, yellow 35px, black 30px, black 60px);
}

HTML

<div class='alert'>
    ....
</div>

Demo