.bar {
height: 10px;
width: 350px;
background: -webkit-linear-gradient(left, red 0%, red 14.28%, orange 14.28%, orange 28.57%, yellow 28.57%, yellow 42.85%, green 42.85%, green 57.14%, blue 57.14%, blue 71.43%, indigo 71.43%, indigo 85.72%, violet 85.72%, violet 100%);
background: red;
background: -o-linear-gradient(left, red 0%, red 14.28%, orange 14.28%, orange 28.57%, yellow 28.57%, yellow 42.85%, green 42.85%, green 57.14%, blue 57.14%, blue 71.43%, indigo 71.43%, indigo 85.72%, violet 85.72%, violet 100%);
background: -moz-linear-gradient(left, red 0%, red 14.28%, orange 14.28%, orange 28.57%, yellow 28.57%, yellow 42.85%, green 42.85%, green 57.14%, blue 57.14%, blue 71.43%, indigo 71.43%, indigo 85.72%, violet 85.72%, violet 100%);
background: linear-gradient(to right, red 0%, red 14.28%, orange 14.28%, orange 28.57%, yellow 28.57%, yellow 42.85%, green 42.85%, green 57.14%, blue 57.14%, blue 71.43%, indigo 71.43%, indigo 85.72%, violet 85.72%, violet 100%);
}
<div class="bar"></div>