mdn docs sticky sidenav proposal styles...

mdnDocsStickySideNav.css

/* check this demo video out */
/* http://cl.ly/nxz6 */

@media screen and (min-height: 42.5em) and (min-width: 47.9385em) {
  @supports (position: sticky) {
    .toc {
      height: 100vh;
      width: 18em;
      overflow: auto;
    }
    .toc + .clear {
      margin-left: 18em;
      margin-top: -100vh;
    }
    .toc-head {
      padding-top: 20px;
    }
    .toc-links {
      display: block;
    }
    .toc-links a {
      white-space: normal;
    }
  }
}