HTML
<a class="banner-twitter" href="http://twitter.com/joaodasilva">Siga-me no Twitter</a>
CSS
.banner-twitter {
position: absolute;
top: 0;
right: 0;
line-height: 1;
/* largura = 14em */
/* altura = 3.5em */
/* tx = .707 * h - .707 * w + w */
/* ty = -.707 * h */
-webkit-transform: translate(6.5765em, -2.4745em) rotate(45deg);
transform: translate(6.5765em, -2.4745em) rotate(45deg);
-webkit-transform-origin: 0 0;
transform-origin: 0 0;
width: 14em;
padding: 1em 0;
box-sizing: border-box;
text-align: center;
text-decoration: none;
background-color: #3C1D3D;
color: white;
border: .25em solid black;
font-size: 1.2em;
font-family: "Open Sans Condensed", sans-serif;
}