JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">

<head>
    <title>HTML5/CSS3 Responsive Theme</title>
    <link rel="stylesheet" type="text/css" href="style.css" />
    <META http-equiv="Content-Type" content="text/html; charset=utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
</head>

<body class="body">

    <header class="mainHeader">
        <img src="emweb1.png">

        <nav>
            <ul>
                <li class="active"><a href="#">Home</a></li>
                <li><a href="#">About</a></li>
                <li><a href="#">Portfolio</a></li>
                <li><a href="#">Contact</a></li>
            </ul>
        </nav>
    </header>

    <div class="container">
    <div class="mainContent">
        <div class="content">
            <article class="topcontent">
                <header>
                    <h2><a href="#" title="first post">First post</a>
                    </h2>
                </header>

                <footer>
                    <p class="post-info">This post is written by Top na webe</p>
                </footer>

                <content>
                    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum
                    </p>
                    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat...

CSS

body{
	background-image: url('img/bg.png');
	color: #000305;
	font-size: 87,5%;
	font-family: Arial, 'Lucida Sans Unicode';
	line-height: 1,5;
	text-align: left;
    display: flex;
    flex-direction: row; /* dame stranku na polovicu */
}

a{
	text-decoration: none;
}

a:link, a:visited{
	color: #CF5C3F;
}

a:hover, a:active{
	color: white;
	background-color:: 

}
.body{

	clear: both;
}

.mainHeader {
    display: flex;
    flex-direction: column;
}
.mainHeader img{
	width: 40%;
	height: auto;
	margin: 2% 0;
}


.mainHeader nav{
	
	background-color: #666;
	height: 40px;
	border-radius: 5px;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
}

.mainHeader nav ul{
list-style: none;
margin: 0 auto;

}

.mainHeader nav ul li{
	float: left;
	display: inline;
	
}


.mainHeader nav a:link, .mainHeader nav a:visited{
	color: #FFF;
	display: inline-block;
	padding: 10px 25px;
	height: 20px;
	
}

.mainHeader nav a:hover, .mainHeader nav a:active,
.mainHeader nav .active a:link, .mainHeader nav .active a:visited{
	background-color: #CF5C3F;
	text-shadow:none;
}

.mainHeader nav ul li a {
	border-radius: 5px;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
}

.mainContent {
	line-height: 25px;
	border-radius: 5px;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
    display: flex;
    flex-direction: column;
	
}

.content{
	width: 70%;
	float: left;
    display: flex;
    flex-direction: column;

}

.topcontent{
	background-color: grey;
	border-radius: 5px;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
	padding: 3% 5%;
	margin-top: 2%;
}

.bottomcontent{
	background-color: grey;
	border-radius: 5px;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
	padding: 3% 5%;
	margin-top: 2%;
}

.top-sidebar {
	width: 21%;
	float: left;
	background-color: grey;
	border-radius: 5px;
	-moz-border-radius: 5px;
	-webkit-border-radius: 5px;
	margin: 2% 0 2% 3%;
	padding: 2% 3%
}

.post-info{
	font-style: italic;
	color: #fff;
	font-size:...