Answers for "css transition underline on hover"

add expanding underline on page load css

#sec {
  display: inline-block;
  position: relative;
  padding-bottom: 3px;
}
#sec:after {
  content: '';
  display: block;
  margin: auto;
  height: 3px;
  width: 0px;
  background: transparent;
  transition: width .5s ease, background-color .5s ease;
}
#sec:hover:after {
  width: 100%;
  background: blue;
}

text decoration css transition

.un {
  display: inline-block;
}

.un::after {
  content: '';
  width: 0px;
  height: 1px;
  display: block;
  background: black;
  transition: 300ms;
}

.un:hover::after {
  width: 100%;
}