Test underlines on links with background-image http://sassmeister.com/gist/f759d03fa1e97727923f

SassMeister-input-HTML.html

<a href="#">Test Link</a>

SassMeister-input.scss

// ----
// Sass (v3.3.7)
// Compass (v1.0.0.alpha.18)
// ----

$color-link: #000;
body {
  font-family: sans-serif;
}
a {
  font-size: 2em;
  color: #5d7699;
  text-decoration: none;
  background-image: linear-gradient(to bottom, rgba(0, 0, 0, 0) 50%, rgba(93, 118, 153, 0.5) 50%);
  background-repeat: repeat-x;
  background-size: 1px 1px;
  background-position: 0 1.2em;
}

SassMeister-output.css

body {
  font-family: sans-serif;
}

a {
  font-size: 2em;
  color: #5d7699;
  text-decoration: none;
  background-image: linear-gradient(to bottom, transparent 50%, rgba(93, 118, 153, 0.5) 50%);
  background-repeat: repeat-x;
  background-size: 1px 1px;
  background-position: 0 1.2em;
}

SassMeister-rendered.html

<a href="#">Test Link</a>