gistfile1.css

/* 1px hairline, using a GIF  */
background: repeat-x bottom left 
  url(data:image/gif;base64,R0lGODlhAQACAPABAMjHzP///yH/C1hNUCBEYXRhWE1QAz94cAAh+QQFAAABACwAAAAAAQACAAACAgwKADs=);
background-size: 100% 1px;

/* 1px hairline, using SVG (background-size property not required) */
background: repeat-x bottom left
  url("data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' width='1' height='1'><rect fill='#ff0000' x='0' y='0' width='1' height='0.5'/></svg>");