First few lines from navbar.less from Twitter Bootstrap http://twitter.github.com/bootstrap/

navbar.less

.navbar {
  // Fix for IE7's bad z-indexing so dropdowns don't appear below content that follows the navbar
  *position: relative;
  *z-index: 2;

  overflow: visible;
  margin-bottom: @baseLineHeight;
}

// Gradient is applied to it's own element because overflow visible is not honored by IE when filter is present
.navbar-inner {
  padding-left:  20px;
  padding-right: 20px;
  #gradient > .vertical(@navbarBackgroundHighlight, @navbarBackground);
  .border-radius(4px);
  @shadow: 0 1px 3px rgba(0,0,0,.25), inset 0 -1px 0 rgba(0,0,0,.1);
  .box-shadow(@shadow);
}