<p class="line">
  <span>Clé</span>
  <span>Valeur</span>
</p>

.line {
  overflow: hidden; /* bloque le dépassement des flottants */
  background: url(...) repeat-x left 1em;
}
.line > :first-child {
  float: left;
  max-width: 40%;
  padding: 0 10px 4px 0;
  background: white;
}
.line > :first-child {
  float: right;
  max-width: 40%;
  padding: 0 0 4px 10px;
  background: white;
}