Вы ищете что-то вроде это ?
.mainContentSection {
margin-top: 10px;
color: white;
display: flex;
overflow: hidden; /* I added this value*/
}
Вот полный код.Я также добавил немного текста в раздел G, чтобы прояснить ситуацию.
.clearFix {
clear: both;
}
/* Header blocks */
.headerBlockSection {
color: white;
font-size: 50px;
}
.block-A {
width: calc((100% / 4 * 3) - 10px);
height: 200px;
background-color: #008000;
float: left;
margin-right: 10px;
}
.block-B {
width: calc(100% / 4 * 1);
height: 200px;
background-color: #008000;
float: left;
}
/* Main content sections */
.mainContentSection {
margin-top: 10px;
color: white;
display: flex;
overflow: hidden; /* I added this value*/
}
.contentSection-1 {
width: calc((100% / 4 * 1));
float: left;
height: auto;
}
.contentSection-2 {
width: calc((100% / 4 * 2) - 20px);
float: left;
margin-left: 10px;
margin-right: 10px;
}
.contentSection-3 {
width: calc((100% / 4 * 1));
float: left;
}
/* Left */
.block-C {
background-color: #f8eb3a;
height: 100%;
}
/* Middle */
.block-D {
height: 150px;
background-color: #316266;
}
.block-E {
margin-top: 10px;
height: 150px;
background-color: #316266;
}
.block-G {
margin-top: 10px;
background-color: #316266;
height: 100%;
}
/* Right */
.block-F {
height: 150px;
background-color: #801a00;
}
.block-H {
margin-top: 10px;
background-color: #801a00;
height: 100%;
}
/* Footer blocks */
.block-I {
margin-top: 10px;
width: 100%;
height: 200px;
background-color: #9ad7ff;
}
<body>
<header class="headerBlockSection">
<div class="block-A">A</div>
<div class="block-B">B</div>
<div class="clearFix"></div>
</header>
<section class="mainContentSection">
<div class="contentSection-1">
<div class="block-C"> C </div>
</div>
<div class="contentSection-2">
<div class="block-D">D</div>
<div class="block-E">E</div>
<div class="block-G"> G <br> Lorem ipsum dolor sit amet consectetur adipisicing elit. Numquam, quas ullam ad eveniet ratione maiores fuga sapiente alias repellendus dolor sequi voluptate quaerat aliquam ut totam, perspiciatis cupiditate molestias? Tempora!
</div>
</div>
<div class="contentSection-3">
<div class="block-F">F</div>
<div class="block-H">H</div>
</div>
<div class="clearFix"></div>
</section>
<div class="block-I">I</div>
</body>