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HTML 

              
                <!-- do we really need to bother with IE9 now? -->
<!--[if lte IE 9]>
<style>
ul.text-slide a:after{display:none}
ul.text-slide a:hover{
	top:-25px;
	background:#eee;color:#444;
}
ul.text-slide a:hover b{color:#444}
</style>
<![endif]-->

<body id="bodytop">
  <div id="toggle" class="hamburger"> <a class="menu-open" href="#toggle"> <span class="toggle"><i>Menu open</i></span> </a> <a class="menu-close" href="#bodytop" id="toggle2"> <span class="toggle"><i>Menu Close</i></span> </a> </div>
  <div class="wrap">
    <header class="header">
      <h1>My&nbsp;Logo/Name</h1>
      <nav class="main-nav">
        <ul class="text-slide">
          <!-- the nested  'b' element is just used for the text-slide effect and not needed for the centering demo -->
          <li><a href="#"><b>Link 1</b></a></li>
          <li><a href="#"><b>Link 2</b></a></li>
          <li><a href="#"><b>Link 3</b></a></li>
          <li><a href="#"><b>Link 4</b></a></li>
          <li><a href="#"><b>Link 5</b></a></li>
          <li><a href="#"><b>Link 6</b></a></li>
          <li><a href="#"><b>Link 7</b></a></li>
        </ul>
      </nav>
      <p class="sign-reg"><a class="sign-in" href="#">Sign In</a> <a class="register" href="#">Register</a></p>
    </header>
    <main class="main">
      <h2>Main content here</h2>
      <p>The main nav links above will remain centred in the viewport even though the logo and sign-in links are uneven lengths. The main caveat is that the nav links will not be allowed to wrap as white-space:nowrap is used to force them onto one line.
        Therefore you will need to use a media query to change the behaviour at about 860px or whatever your layout requires.</p>
    </main>
    <footer class="footer">
      <h3>Footer content here</h3>
    </footer>
  </div>
</body>
!

CSS 

              
                html {
  box-sizing: border-box;
}
*,
*:before,
*:after {
  box-sizing: inherit;
}
html,
body {
  margin: 0;
  padding: 0;
  height: 100%;
}
body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica,
    Arial, sans-serif;
}
.wrap {
  background: #f9f9f9;
  max-width: 1170px;
  margin: auto;
  display: flex;
  flex-direction: column;
  min-height: 100%;
}
.header {
  display: flex;
  padding: 10px;
  background: #eee;
  align-items: center;
  border-bottom: 2px solid #000;
}
.header h1 {
  margin: 0;
  padding: 0 10px 0 0;
}
.main-nav {
  flex: 1 0 0%;
  white-space: nowrap;
  transition: 0.5s ease;
}
.header h1,
.sign-reg {
  flex-basis: 50%;
}
.main-nav ul {
  margin: 0;
  padding: 0;
  list-style: none;
  display: flex;
}
.main-nav a {
  display: block;
  padding: 10px;
  color: #fff;
  background: #000;
  text-decoration: none;
  border: 1px solid #fff;
}
/* code below just for a sliding effect on the nav links and is not needed for main demo */
ul.text-slide {
  height: 2rem;
  font-family: Arial, Helvetica, sans-serif;
  font-size: 0.8rem;
  margin: 0;
  padding: 0;
  list-style: none;
}
ul.text-slide li {
  border: 1px solid #444444;
  border-right: 1px solid #fff;
  height: 2rem;
  overflow: hidden;
}
ul.text-slide li:last-child {
  border-right: 1px solid #444;
}
ul.text-slide a {
  padding: 0;
  border: none;
  position: relative;
  top: -2rem;
  padding-top: 2rem;
  text-decoration: none;
  background: #444;
  -moz-transition: top 0.2s linear;
  -webkit-transition: top 0.2s linear;
  transition: top 0.2s linear;
}
ul.text-slide a:after {
  content: " ";
  background: #eee;
  position: absolute;
  top: 0px;
  left: 0;
  right: 0;
  height: 2rem;
  z-index: 2;
}
ul.text-slide a:hover {
  top: 0;
}
ul.text-slide a b {
  outline: none;
  padding: 0 15px;
  display: block;
  position: relative;
  z-index: 3;
  font-weight: normal;
  text-shadow: 0 -2rem 0px #444;
  color: #fff;
  height: 2rem;
  line-height: 2rem;
}
/* end text effect code */
p.sign-reg {
  margin: 0;
  padding: 0 0 0 10px;
  display: flex;
  justify-content: flex-end;
}
.sign-reg a {
  padding: 10px;
  white-space: nowrap;
}
.main {
  background: #ccc;
  padding: 10px;
  flex: 1 0 0%; /* remove this if you don't want main's background to reach footer*/
}
.footer {
  background: #000;
  padding: 10px;
  color: #fff;
  margin-top: auto;
}
.hamburger {
  display: none;
}
/* media queries for smaller display */

@media screen and (max-width: 860px) {
  .wrap {
    margin: 0;
  }
  .header {
    padding-left: 40px;
  }
  .header h1 {
    padding-left: 10px;
  }
  .main-nav {
    position: fixed;
    left: 0;
    top: 0;
    bottom: 0;
    width: 100%;
    z-index: 999;
    background: rgba(255, 255, 255, 0.9);
    overflow: auto;
    flex-wrap: wrap;
    transform: translateX(-100%);
  }
  .main-nav ul {
    height: auto;
    flex-wrap: wrap;
  }
  .main-nav li {
    flex: 1 0 100%;
  }
  /* hamburger styles */
  .hamburger {
    position: fixed;
    display: block;
    left: 10px;
    top: 15px;
    z-index: 1000;
    transition: 0.5s ease;
    width: 35px;
    height: 35px;
    background:blueviolet;
    transition-delay: 0.5s;
  }
  .hamburger a {
    background: transparent;
    color: transparent;
    position: absolute;
    top: 0;
    left: 1px;
    width: 32px;
    height: 35px;
    overflow: hidden;
  }
  /* hamburger menu */
  .toggle {
    position: absolute;
    top: 7px;
    left: 3px;
    background: transparent;
    padding: 0;
    margin: 0;
    cursor: pointer;
    outline: none;
    width: 26px;
    height: 21px;
    border: none;
    border-top: 3px solid #fff;
    border-bottom: 3px solid #fff;
    text-decoration: none;
    color: #ccc;
    outline: 0;
  }
  .toggle i {
    opacity: 0;
  }
  .toggle:before,
  .toggle:after {
    content: "";
    display: block;
    width: 26px;
    height: 3px;
    top: 6px;
    left: 0;
    background: #fff;
    position: absolute;
  }
  .toggle,
  .toggle:before,
  .toggle:after {
    transition: all 0.3s ease;
    transform: rotate(0deg);
  }
  .hamburger:target .toggle {
    border-color: transparent;
  }
  .hamburger:target .toggle:before {
    transform: rotate(45deg);
  }
  .hamburger:target .toggle:after {
    transform: rotate(-45deg);
  }
  .menu-close {
    z-index: -1;
  }
  .hamburger:target .menu-open {
    opacity: 0;
  }
  .hamburger:target .menu-close {
    z-index: 2;
  }
  /* nav styles open */
  .main-nav {
    padding-top: 45px;
  }
  .main-nav li,
  .main-nav li a {
    background: transparent;
    border: none;
  }
  ul.text-slide {
    height: auto;
    font-size: 1.5rem;
  }
  ul.text-slide li {
    border: none;
  }
  .hamburger:target + .wrap .main-nav {
    transform: translateX(0);
    background: rgba(255, 255, 255, 1);
    transition-delay: 1s;
  }
  .hamburger:target {
    left: 92%;
    transition-delay: 0.5s;
  }
  .hamburger:target + .wrap .main-nav a,
  .hamburger:target + .wrap ul.text-slide li {
    border-bottom: 1px solid #000;
  }
  ul.text-slide {
    height: auto;
  }
  ul.text-slide li {
    border: none;
    border-bottom: 1px solid #000;
    height: 3rem;
  }
  ul.text-slide li:last-child {
    border-right: none;
  }
  ul.text-slide a {
    top: -3rem;
    padding-top: 3rem;
    font-size: 1.5rem;
    color:#000;
  }
  ul.text-slide a:after {
    height: 3rem;
  }
  ul.text-slide a b {
    text-shadow: 0 -3rem 0px #444;
    height: 3rem;
    line-height: 3rem;
    color:#000;
  }
  /* end text effect code */
}

@media screen and (max-width: 480px) {
  h1 {
    font-size: 1rem;
  }
}
!

JS 

              
                //nope
!
999px

Console