Default stylesheet I use for rendering markdown into html based on my lsr.css for restructuredText.

md.css

body {
  font-family: 'Helvetica Neue', sans;
  background: #ffffff;
  color: #222;
  margin: 2em auto;
  padding: 0 2em;
  width: 48em;
  line-height: 1.5em;
  font-size: 16px;
  font-weight: 300;
}

img {
  margin: 0;
  border: 0;
}

p {
  margin: 1em 0;
}

a {
  color: #00213D;
}

a:visited {
  color: #00213D;
  background-color: transparent;
}

a:active {
  color: #318100;
  background-color: transparent;
}

a:hover {
  text-decoration: none;
}

p img {
  border: 0;
  margin: 0;
}

h1, h2, h3, h4, h5, h6 {
  color: #003a6b;
  background-color: transparent;
  margin: 1em 0;
  font-weight: normal;
}

h1 {
  font-size: 180%;
}

h2 {
  font-size: 160%;
}

h3 {
  font-size: 140%;
}

h4 {
  font-size: 110%;
}

h5 {
  font-size: 105%;
}

h6 {
  font-size: 100%;
}

dt {
  font-style: italic;
}

dd {
  margin-bottom: 1.5em;
}

li {
  line-height: 1.5em;
}

code {
  padding: 0.1em;
  font-size: 14px;
  font-family: "Menlo", monospace;
  background-color: #f5f5f5;
  border: 1px solid #efefef;
}

pre {
  font-family: "Menlo", monospace;
  background-color: #fff;
  padding: 0.5em;
  line-height: 1.25em;
  border: 1px solid #efefef;
  border-bottom: 1px solid #ddd;
  -webkit-box-shadow: 0 1px 3px 0 #eee;
  -moz-box-shadow: 0 1px 3px 0 #eee;
  -ms-box-shadow: 0 1px 3px 0 #eee;
  box-shadow: 0 1px 3px 0 #eee;
}

pre code {
  background-color: transparent;
  border-width: 0;
}

blockquote {
  border-top: 1px solid #efefef;
  border-bottom: 1px solid #ddd;
  -webkit-box-shadow: 0 1px 3px 0 #eee;
  -moz-box-shadow: 0 1px 3px 0 #eee;
  -ms-box-shadow: 0 1px 3px 0 #eee;
  box-shadow: 0 1px 3px 0 #eee;
}

table {
  border-collapse: collapse;
  border: 1px solid #efefef;
  border-bottom: 1px solid #ddd;
  -webkit-box-shadow: 0 1px 3px 0 #eee;
  -moz-box-shadow: 0 1px 3px 0 #eee;
  -ms-box-shadow: 0 1px 3px 0 #eee;
  box-shadow: 0 1px 3px 0 #eee;
}

td, th {
  border: 1px solid #ddd;
  padding: 0.5em;
}

th {
  background-color: #f5f5f5;
}