html {
    background-color: lightblue;
    color: #000;
<!-- 2 -->

<style>
#important_wrapper { /* id */
  margin-left: 10px;
  border-left: 3px solid red;
  padding-left: 20px;
}
.important { /* classe */
  font-weight: bolder;
  color: red;
}
.pas_important {
  font-weight: normal;
}
</style>