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HTML 

              
                <link href='//fonts.googleapis.com/css?family=Lato' rel='stylesheet' type='text/css'>
<header class="header">
			<!-- Logo -->
			<a class="logo" href="https://www.savetherhino.org" title=" font-family: Lato, sans-serif">RHINO</a>
  <nav><!-- MAIN MENU <div class="navicon">&#9776;</div> <div class="navicon">?</div> -->
				<input type="checkbox" id="show-menu-input-checkbox" role="button">
				<label for="show-menu-input-checkbox" id="main-menu-toggler" class="flex-container flex-vcenter flex-hcenter"><div id="main-menu-toggler-icon" class="flex-grow"></div></label>
				<ul class="mainMenu accordion" id="mainMenu"><li><a href="https://commons.wikimedia.org/wiki/Kunstformen_der_Natur" target="_blank">life</a></li><li><a href="https://en.wikipedia.org/wiki/Douglas_Adams" target="_blank">universe</a></li><li><a href="https://archive.org/details/internetarcade" target="_blank">fun</a></li><li><a href="https://archive.org/details/prelinger" target="_blank">archive</a></li><li><a href="https://www.katetempest.co.uk" target="_blank">fight</a></li><li><a href="https://no.wikipedia.org/wiki/Siv_(mytologi)" target="_blank">love</a></li><li><a href="http://www.dankaplantrio.com" target="_blank">waves</a></li><li><a href="https://edwardsnowden.substack.com/" target="_blank">happening</a></li></ul>
  </nav>
</header>
<main><p>The toggle animation is forked from <a href="https://codepen.io/CreativeJuiz/">Geoffrey Crofte</a>'s nice <a href="https://codepen.io/CreativeJuiz/pen/oCBxz?editors=0010">'Menu Toggle button with flat menu' pen</a>.</p>
  <p>The CSS only stuff is based off <a href="https://medialoot.com/blog/how-to-create-a-responsive-navigation-menu-using-only-css">Tony Thomas' article at medialoot</a> and <a href="http://stackoverflow.com/questions/18752134/reveal-and-hide-a-div-on-checkbox-condition-with-css">a question at stackoverflow</a>. It's true, though, <a href="https://css-tricks.com/responsive-menu-concepts/">as pointed out at CSS tricks</a>, that it's bad semantics, but it's quite a simple solution, anyhow.</p>
  <p>These pages were a great help to get a grip on flexbox magic:</p>
  <ul>
    <li><a href="http://www.heydonworks.com/article/flexbox-grid-finesse">http://www.heydonworks.com/article/flexbox-grid-finesse</a></li>
    <li><a href="https://codepen.io/heydon/pen/GpbQdP">https://codepen.io/heydon/pen/GpbQdP</a></li>
    <li><a href="https://philipwalton.github.io/solved-by-flexbox/">https://philipwalton.github.io/solved-by-flexbox/</a></li>
    <li><a href="http://apps.workflower.fi/css-cheats/?name=flexbox">http://apps.workflower.fi/css-cheats/?name=flexbox</a></li>
  </ul>
</main>
!

CSS 

              
                /* 
  RESETS
/*-----------------------------------------------------*/
html{
  box-sizing: border-box;
  width:100%;height:100%;
  margin:0;padding:0;
  font-family: Lato, sans-serif;
}
body{
  width:100%;height:100%;margin:0;padding:0;
  border:1px dotted gray;
}
*, *:before, *:after {
	box-sizing: inherit;
}
header ul, menu, dir {
    margin-block-end: 0;
    margin-block-start: 0;
    padding-inline-start: 0;
    -webkit-padding-start: 0;
}
a{text-decoration:none;color:rgba(255,102,0,1);}

/* 
  LAYOUT
/*-----------------------------------------------------*/
:root{
	--maxSiteWidth:1080px;
}
main{
  margin: 90px auto;padding:1em;
}
header {
    display: flex;
    flex-flow: row nowrap;
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    background: rgba(255,255,255,1);
    height: 80px;
    border-bottom: rgba(44,44,44,.6) 1px solid;
    margin: 0 auto 0 auto;
}
.logo {
    height:80px;
    flex: 0 0 200px;
    font-size:42px;display:block;text-align:center;align-items:center;justify-content:center;margin:14px auto;
}

/*  
   MAIN MENU MOBILE FIRST
/*-------------------------------------------------------*/
nav{flex:1 auto;}
.mainMenu{
  position: fixed;top:0;left: 0;right: 0;
  margin:80px auto 0 auto;
	height:90%;/**hack for opera mini**/
	height:calc(100% - 79px);
  width:100%;
  flex-flow:column nowrap;
  align-content:stretch;
  display:none;
}
.mainMenu a{
  flex:1;
  width:100%;
  justify-content: center;
  align-items:center;
  font-size:26px;
  display:flex;
  background:transparent;
}
.mainMenu>li {
	flex:1;
  display:flex;
  flex-flow:column nowrap;
  align-items:center;
  text-align:center;
	border-bottom: 1px dotted rgba(22,22,22,.3);
  background:rgba(255,255,255,1);
}

/*  
  MAIN MENU TOGGLE BUTTON
/*-----------------------------------------------------*/
/*Hide checkbox*/
#show-menu-input-checkbox{
	display: none;
}
/*Show menu when invisible checkbox is checked*/
#show-menu-input-checkbox:checked ~ .mainMenu{
	display: flex;
}
#main-menu-toggler {
  width:80px;
  height:80px;
  position:fixed;
  top:0;
  right:0;
  padding:24px;
  display:flex;
  justify-content:center;align-items:center;
}
#main-menu-toggler-icon:after,
#main-menu-toggler-icon:before {
content: "";
position: absolute;
left: 0;
top: -9px;
}
#main-menu-toggler-icon:after{
top: 9px;
}
#main-menu-toggler-icon {
position: relative;
display: block;
}
#main-menu-toggler-icon,
#main-menu-toggler-icon:after,
#main-menu-toggler-icon:before {
width: 100%;
height: 5px;
background-color:rgba(22,22,22,1);
transition: all 0.3s;
backface-visibility: hidden;
border-radius: 2px;
}
/* on menu activation */
#show-menu-input-checkbox:checked ~ #main-menu-toggler>#main-menu-toggler-icon {
background-color: transparent;
}
#show-menu-input-checkbox:checked ~ #main-menu-toggler>#main-menu-toggler-icon:before {
transform: rotate(45deg) translate(5px, 5px);
}
#show-menu-input-checkbox:checked ~ #main-menu-toggler>#main-menu-toggler-icon:after {
transform: rotate(-45deg) translate(7px, -8px);
}

/*
  MAIN MENU ON WIDE SCREENS
/*----------------------------------------------------------*/
@media only screen and (min-width: 1080px) {
	header,.mainMenu,nav,main,article,footer{max-width:var(--maxSiteWidth);}
	/**HIDE THE MENU TOGGLER**/
	#main-menu-toggler{display:none;}
	/**APPLY HORIZONTAL BAR STYLING TO THE MAIN MENU**/
	.mainMenu{
    position: initial;
    display:flex;
    flex-flow:row nowrap;
    flex-direction: row;
    justify-content: flex-end;
    height:80px;
    margin-top:0;
  }
	.mainMenu>li {
    flex:1 0 auto;
    border-bottom:none;
    background:transparent;
    align-items:initial;
  }
	.mainMenu a{
    font-size:22px;
    width:auto;
    justify-content: flex-end;
    padding-left:16px;
  }
}
!

JS
!
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