<p>word-breakなし</p>
<div>
  <a href="http://www.thelongestdomainnameintheworldandthensomeandthensomemoreandmore.com/">http://www.thelongestdomainnameintheworldandthensomeandthensomemoreandmore.com/</a>
</div>

<p>word-break適応</p>
<div class="word-break">
  <a href="http://www.thelongestdomainnameintheworldandthensomeandthensomemoreandmore.com/">http://www.thelongestdomainnameintheworldandthensomeandthensomemoreandmore.com/</a>
</div>

div {
  border: solid 1px #999;
  width: 300px;
  padding: 10px;
  margin-bottom: 30px;
}
div.word-break {
  word-wrap: break-word;
}