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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

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CSS

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CSS Base

About CSS Base

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HTML 

              
                <div id="body-in">
	<div id="main">
		<p>アーカイブをそれぞれの年ごとに表示するようにしました</p>
		<p>最新の年のリスト一覧は表示するようにしてあります。それ以外はデフォルトで非表示。それぞれの年をクリックするとその年の投稿記事一覧が表示される仕組みです。</p>
		<p>最大で横3列4段になります。<br>それぞれの月、カウント(そのpostにも)classが付けてあるので、色やら変更可能です。<br>逆にカウントが無いと上手く機能しないかも知れないので、デフォルトでカウント表示、ドロップダウンリスト<strong>ではなく</strong>通常表示にしておいて下さい</p>
		<p>多分これで過去10年までなら、本来エライことになる縦スペースを多少確保できたと思います。<br>そういう場合は、ドロップダウンリストを使うべきなんでしょうけれども。</p>
	</div>
</div>
<footer itemscope="" itemtype="http://schema.org/WPFooter">
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			<div id="footer-widget">
				<div class="footer-left">
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						<h3 class="footer_widget_title">固定ページ</h3>
						<ul>
							<li class="page_item page-item-17"><a href="https://wp-simplicity.com/">Simplicityの特徴</a></li>
							<li class="page_item page-item-7 page_item_has_children"><a href="https://wp-simplicity.com/downloads/">Simplicityのダウンロード</a>
								<ul class="children">
									<li class="page_item page-item-1339"><a href="https://wp-simplicity.com/downloads/child-theme/">Simplicityの子テーマ</a></li>
									<li class="page_item page-item-35004"><a href="https://wp-simplicity.com/downloads/downloads2/">Simplicity2のダウンロード</a></li>
									<li class="page_item page-item-35010"><a href="https://wp-simplicity.com/downloads/child-theme2/">Simplicity2の子テーマ</a></li>
									<li class="page_item page-item-1877"><a href="https://wp-simplicity.com/downloads/skin/">Simplicityのスキン</a></li>
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							<li class="page_item page-item-43"><a href="https://wp-simplicity.com/initialization/">Simplicityの初期設定・おすすめの設定</a></li>
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									<li class="page_item page-item-1143"><a href="https://wp-simplicity.com/reference/adoption-sites/">Simplicity導入サイト</a></li>
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							<li class="page_item page-item-9262 page_item_has_children"><a href="https://wp-simplicity.com/forums/">Simplicityサポートフォーラム</a>
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									<li class="page_item page-item-9258"><a href="https://wp-simplicity.com/forums/create-topic/">トピックの作成</a></li>
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									<li class="cat-item cat-item-13"><a href="https://wp-simplicity.com/category/settings/extension-class/" title="Simplicityの文章を彩る拡張クラスの紹介です。">拡張クラス</a> (2)
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							<li class="cat-item cat-item-6"><a href="https://wp-simplicity.com/category/customizer/" title="Wordpress管理画面からできるカスタマイズWordpress管理画面から「外観→カスタマイズ」で行える設定に関する情報です。">カスタマイザー設定</a> (30)
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							<li class="cat-item cat-item-4"><a href="https://wp-simplicity.com/category/theme-custom/" title="テーマファイルのカスタマイズ方法などSimplicityのテーマファイルを編集してカスタマイズする方法の紹介です。テーマファイルのカスタマイズによる不具合、それに関する質問は、サポートできませんのでご了承ください。">テーマ改造</a> (34)
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					<aside id="archives-4" class="widget widget_archive">
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							<li><a href="https://wp-simplicity.com/2016/01/">2016年1月</a>&nbsp;(3)</li>
							<li><a href="https://wp-simplicity.com/2015/12/">2015年12月</a>&nbsp;(6)</li>
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							<li><a href="https://wp-simplicity.com/2015/02/">2015年2月</a>&nbsp;(10)</li>
							<li><a href="https://wp-simplicity.com/2015/01/">2015年1月</a>&nbsp;(13)</li>
							<li><a href="https://wp-simplicity.com/2014/12/">2014年12月</a>&nbsp;(14)</li>
							<li><a href="https://wp-simplicity.com/2014/11/">2014年11月</a>&nbsp;(17)</li>
							<li><a href="https://wp-simplicity.com/2014/10/">2014年10月</a>&nbsp;(18)</li>
							<li><a href="https://wp-simplicity.com/2014/09/">2014年9月</a>&nbsp;(19)</li>
							<li><a href="https://wp-simplicity.com/2014/08/">2014年8月</a>&nbsp;(31)</li>
							<li><a href="https://wp-simplicity.com/2014/07/">2014年7月</a>&nbsp;(33)</li>
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			<div class="clear"></div>
			<div id="copyright" class="wrapper">
				© 2014 <a href="https://wp-simplicity.com">Simplicity</a>
			</div>
		</div>
		<!-- /#footer-in -->
	</div>
	<!-- /#footer -->
</footer>
!

CSS 

              
                #footer {
	.widget_archive {
		display: none;
	}
	
	.archive2 {
		header {
			position: relative;
			&::before{
				content:"clickで表示・非表示トグル";
				position: absolute;
				top: 50%;
				right: -500px;
				transform: translateY(-50%);
				font-size: 10px;
				color: #fff;
				transition: .3s ease;
				background-color: #833;
				padding: 0 2px;
			}
			&:hover::before {
				right: 0;
			}
		}
		
		ul {
			display: flex;
			flex-flow: row wrap;
			transition: .3s ease;
			height: 100%;
			opacity: 1;
			visibility: visible;
			padding: 0;
			
			li {
				flex: 1 27%;
				margin: 2px 5px;
				padding: 3px;
				background-color: #fff;
				color: #333;
				border-radius: 3px;
				transition: .3s ease;
				font-size: 12px;
				
				&:hover {
					background-color: #333;
					color: #fff;
					box-shadow: 0 0 2px rgba(255,255,255,.8);
				}
				
				a{
					display: flex;
					flex-flow: row wrap;
					justify-content: space-between;
					text-decoration: none;
					color: inherit;
					font-size: inherit;
					
					div:first-child {
						margin-right: auto;
					}
					div:last-child {
						span {
							font-size: 10px;
						}
					}
				}
			}
		}
		
		.hide {
			margin: 0;
			height: 0;
			opacity: 0;
			visibility: hidden;
			overflow: hidden;
		}
	}
}

/*以下不要*/
footer {
	width: 100%;
	#footer-in {
		width: 100%;
		
		#footer-widget {
			display: flex;
			flex-flow: row wrap;
			margin: 0;
			padding: 0;
			justify-content: space-around;
			width: 100%;
			
			[class^=footer-] {
				flex: 1 30%;
				float: none;
				width: initial;
				margin: 5px;
				
				&:last-child {
					min-width: 33%;
				}
				
			}
		}
	}
}
!

JS 

              
                (function($) {
	$(function() {
		var
			wgt = $("footer .widget_archive"),
			clone = wgt.clone(),
			year = [];

		//クローンして、手前に挿入。オリジナルはcssで非表示
		wgt.before(clone);
		$("footer .widget_archive:first").attr("class", "archive2");

		var
			acv = $("footer").find(".archive2"),
			acvLi = acv.find("li");

		//liのテキストから年がどこからあるかを調べる
		acvLi.each(function(i) {
			var dt = $(this).text().match(/(\d+)年(\d+)月\s\((\d+)\)/);
			year.push(dt[1]);
		});
		$.unique(year); //重複削除

		acvLi.unwrap(); //liの親のulを解除

		//投稿年があるだけクローンした中にブロックを作る
		var
			yearCount = year.length,
			i = 0;

		while (i < yearCount) {
			acv.append(
				"<div class='year_" + year[i] + "'>" +
				"<header>" + year[i] + "年</header>" +
				"<ul></ul>" +
				"</div>");
			i++;
		}

		//作ったブロック内のulに内容を整形して移動
		//オリジナルのクローンは順番に削除
		var j = 0;
		acvLi.each(function() {
			var dt = $(this).text().match(/(\d+)年(\d+)月\s\((\d+)\)/),
				href = $(this).find("a").attr("href"),
				rTxt = "<li>" +
				"<a href='" + href + "'>" +
				"<div class='month'>" + dt[2] + "月</div>" +
				"<div class='count'>" + dt[3] + "<span>post</span></div>" +
				"</a>" +
				"</li>";

			if (year[j] === dt[1]) {
				$(".year_" + year[j] + " ul").append(rTxt);
				$(this).remove();
			} else {
				j++;
				$(".year_" + year[j] + " ul").append(rTxt);
				$(this).remove();
			}
		});

		//headerの最初以外は.hide
		acv.find("ul:not(:first)").addClass("hide");

		//headerクリックでトグルshow
		acv.find("header").on("click", function() {
			$(this).next().toggleClass("hide");
		});
	});
})(jQuery);
!
999px

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