Q:

Ничего не отображается после одного блока

Всем привет! Подскажите, почему не отображается <div class="about"> (он в самом конце) , при этом , если задать ему класс sale-top, то все отображается, не могу понять, в чем проблема.

body {
  font-size: 0;
}

* {
  margin: 0;
  padding: 0;
}

.header-top a {
  color: #b1b1b1;
}

a {
  cursor: pointer;
  text-decoration: none;
}

.logo span a {
  color: #b1b1b1;
}

.zakaz span a {
  color: #b1b1b1;
}

.phone a {
  color: #242730;
}

.cart-icon-zero span a {
  color: #ffffff;
}

.inner {
  margin-left: auto;
  margin-right: auto;
  padding-left: 15px;
  padding-right: 15px;
  width: 1170px;
}

.header-top {
  color: #b1b1b1;
  background-color: #252730;
  font-size: 11px;
  font-family: "Open Sans", sans-serif, Verdana, Arial;
  padding-top: 6px;
  padding-bottom: 14px;
}

.header-top-left {
  display: inline-block;
}

.header-top-left span img {
  padding-right: 10px;
}

.header-top-right {
  display: inline-block;
  margin-left: 407px;
}

.header-top-left li {
  display: inline-block;
  border-left: 1px solid #30323b;
  padding-left: 15px;
  padding-right: 15px;
}

.header-top-left span {
  border-left: 1px solid #30323b;
  padding-left: 19px;
  padding-right: 19px;
}

nav {
  display: inline-block;
}

.header-top-right .eye,
.health,
.stat {
  background: url(../images/sprite.png) no-repeat;
  display: inline-block;
  width: 26px;
  height: 17px;
}

.header-top-right .eye {
  background-position: 0 -499px;
}

.header-top-right .health {
  background-position: 3px -465px;
}

.header-top-right .stat {
  background-position: 0 -430px;
}

.header-top-right .entrance {
  margin-right: 11px;
}

.header-middle {
  height: 94px;
}

.logo {
  display: inline-block;
  width: 300px;
}

.logo img {
  margin-top: 20px;
}

.logo span {
  font-size: 12px;
  color: #b1b1b1;
}

.input {
  display: inline-block;
  vertical-align: bottom;
  margin-bottom: 10px;
}

input {
  width: 521px;
  height: 43px;
  vertical-align: bottom;
  border: 1px solid #e6e6e6;
  border-style: solid;
  color: #6a6a6a;
  font-style: italic;
  padding-left: 38px;
  background: url(../images/search.png) no-repeat;
  background-position: 15px;
  outline: none;
}

.mini_phones {
  display: inline-block;
  width: 25px;
  color: #b1b1b1;
  font-size: 11px;
  line-height: 14px;
  padding-left: 30px;
  vertical-align: top;
  padding-top: 30px;
}

.phone {
  display: inline-block;
  font-size: 30px;
  vertical-align: top;
  padding-left: 6px;
  padding-top: 27px;
  color: #242730;
  font-family: "Helvetica";
  font-weight: bold;
}

.zakaz {
  display: inline-block;
  font-size: 14px;
  margin-left: -111px;
  color: #b2b2b2;
}

.zakaz img {
  padding-right: 4px;
}

.korzina {
  display: inline-block;
  background: url(../images/select2x2.png) no-repeat;
  width: 37px;
  height: 53px;
}

.cart-icon {
  display: inline-block;
  vertical-align: super;
}

.header-bottom li a {
  color: #ffffff;
}

.cart-icon-zero {
  position: absolute;
  top: 64px;
  right: 115px;
  padding: 3px 0;
  min-width: 24px;
  border-radius: 50%;
  background: #f54550;
  text-align: center;
  font-size: 14px;
  font-weight: 700;
  color: #FFF;
}

.header-bottom li {
  display: inline-block;
  border-left: 1px solid #f95f65;
  border-bottom-width: 10px;
  padding-left: 27px;
  padding-right: 27px;
}

.sales a {
  color: #ffffff;
}

.header-bottom {
  margin-top: 0;
  color: #ffffff;
  background: linear-gradient(to right, #d81b28 0, #ff6e58 100%);
  vertical-align: middle;
  font-family: "HelveticaRegular";
  font-weight: 700;
  text-transform: uppercase;
  padding-top: 6px;
  padding-bottom: 6px;
  font-size: 13px;
}

.header-bottom img {
  vertical-align: middle;
  padding-right: 10px;
}

.sales {
  display: inline-block;
  background-color: #d6333d;
  padding-left: 16px;
  padding-right: 30px;
}

.sale-top {
  display: inline-block;
  padding-top: 29px;
  font-size: 18px;
}

.sale-top .sale1 {
  font-family: "Helvetica";
  font-weight: bold;
  color: #242730;
}

.sale-top .sale2 {
  color: #f54550;
  margin-left: 41px;
  border-bottom: 1px dotted #e6999d;
}

.center-image img {
  width: 100%;
  height: 100%;
}

.gadgets {
  margin-top: 36px;
  text-align: center;
}

.gadgets .item {
  display: inline-block;
  border: 1px solid #f3f3f3;
  width: 266px;
  height: 400px;
}

.gadgets .item_img {
  text-align: center;
  margin-top: 36px;
}

.gadgets .item_stars {
  text-align: center;
  padding-top: 20px;
}

.gadgets .item_text1 {
  text-align: center;
  font-size: 14px;
  padding-top: 20px;
  font-family: "Helvetica";
  color: #252730;
}

.gadgets .item_text2 {
  text-align: center;
  font-size: 14px;
  font-family: "Helvetica";
  color: #252730;
}

.gadgets .item_price {
  padding-top: 21px;
}

.gadgets .item_price .first-price {
  font-size: 30px;
  font-style: italic;
  font-family: "Helvetica";
  font-weight: bold;
  text-shadow: 1px 1px 0 #ffffff, 2px 2px 0 #ccc;
}

.gadgets .item_price .second-price {
  font-size: 14px;
  color: #b1b1b1;
}

.gadgets .item .second-price:before {
  border-bottom: 1px solid #b1b1b1;
  position: absolute;
  content: "";
  width: 4%;
  height: 1.5%;
  transform: rotate(-8deg);
}

.gadgets .item .icons {
  padding-top: 23px;
}

.gadgets .item .icons .icon1,
.icon3 {
  background: url(../images/sprite.png) no-repeat;
  display: inline-block;
  width: 26px;
  height: 17px
}

.gadgets .item .icons .icon1 {
  background-position: 3px -394px;
}

.gadgets .item .icon2 {
  color: #f54550;
  font-size: 11px;
  font-family: "Helvetica";
  border: 1px solid #f54550;
  padding-top: 16px;
  padding-bottom: 16px;
  border-radius: 5px;
  padding-left: 16px;
  padding-right: 20px;
  text-transform: uppercase;
}

.gadgets .item .icon22 {
  color: #ffffff;
  font-size: 11px;
  font-family: "Helvetica";
  border: 1px solid #f54550;
  padding-top: 16px;
  padding-bottom: 16px;
  border-radius: 5px;
  padding-left: 16px;
  padding-right: 20px;
  text-transform: uppercase;
  background-color: #f54550;
  text-align: center;
  vertical-align: bottom;
}

.gadgets .item .icons .icon22 img {
  padding-right: 6px;
  vertical-align: bottom;
}

.gadgets .item .icons .icon2 img {
  padding-right: 6px;
  vertical-align: bottom;
}

.gadgets .item .icons .icon3 {
  background-position: 9px -430px;
}

.banner {
  margin-top: 40px;
}

.devices {
  margin-top: 36px;
  text-align: center;
}

.devices .item {
  display: inline-block;
  border: 1px solid #f3f3f3;
  width: 266px;
  height: 400px;
}

.devices .item_img {
  text-align: center;
  margin-top: 36px;
}

.devices .item_stars {
  text-align: center;
  padding-top: 20px;
}

.devices .item_text1 {
  text-align: center;
  font-size: 14px;
  padding-top: 20px;
  font-family: "Helvetica";
  color: #252730;
}

.devices .item_text2 {
  text-align: center;
  font-size: 14px;
  font-family: "Helvetica";
  color: #252730;
}

.devices .item_price {
  padding-top: 21px;
}

.devices .item_price .first-price {
  font-size: 30px;
  font-style: italic;
  font-family: "Helvetica";
  font-weight: bold;
  text-shadow: 1px 1px 0 #ffffff, 2px 2px 0 #ccc;
}

.devices .item_price .second-price {
  font-size: 14px;
  color: #b1b1b1;
}

.devices .item .second-price:before {
  border-bottom: 1px solid #b1b1b1;
  position: absolute;
  content: "";
  width: 4%;
  height: 1.5%;
  transform: rotate(-8deg);
}

.devices .item .icons {
  padding-top: 23px;
}

.devices .item .icons .icon1,
.icon3 {
  background: url(../images/sprite.png) no-repeat;
  display: inline-block;
  width: 26px;
  height: 17px
}

.devices .item .icons .icon1 {
  background-position: 3px -394px;
}

.devices .item .icon2 {
  color: #ffffff;
  background-color: #f54550;
  font-size: 11px;
  font-family: "Helvetica";
  border: 1px solid #f54550;
  padding-top: 16px;
  padding-bottom: 16px;
  border-radius: 5px;
  padding-left: 16px;
  padding-right: 20px;
  text-transform: uppercase;
}

.devices .item .icons .icon2 img {
  padding-right: 6px;
  vertical-align: bottom;
}

.devices .item .icons .icon3 {
  background-position: 9px -430px;
}

.about {
  display: inline-block;
}
<div class="allwrapper">
  <header>
    <div class="header-top">
      <div class="inner">
        <div class="header-top-left">
          <nav class="menu">
            <ul>
              <li><a href="#">О магазине</a></li>
              <li><a href="#">Доставка</a></li>
              <li><a href="#">Оплата</a></li>
              <li><a href="#">Контакты</a></li>
              <li><a href="#">Помощь</a></li>
            </ul>
          </nav>
          <span><img src="images/clock.png">Работаем с 9 до 21</span>
        </div>
        <div class="header-top-right">
          <a href="#"><span class="eye"></span></a>
          <a href="#"><span class="health"></span></a>
          <a href="#"><span class="stat"></span></a>
          <a href="#"><span class="entrance">Вход</span></a>
          <a href="#"><span>Регистрация</span></a>
        </div>
      </div>
    </div>
    <div class="header-middle">
      <div class="inner">
        <div class="logo">
          <img src="images/logo.png">
          <span><a href="#">Интернет-магазин электроники и бытовой техники</span></a>
        </div>
        <div class="input">
          <form action="#" method="get" name="input">
            <input type="text" placeholder="Поиск по каталогу">
          </form>
        </div>
        <div class="mini_phones">
          <span>(029)</span>
          <span>(033)</span>
        </div>
        <div class="phone">
          <a href="#"><span>640-80-80</span></a>
        </div>
        <div class="zakaz">
          <span><a href="#"><img src="images/phone-mini.png">Заказать звонок</a></span>
        </div>
        <div class="korzina">
        </div>
        <div class="cart-icon">
          <a href="#"><img src="images/cart-icon.png"></a>
        </div>
        <div class="cart-icon-zero">
          <span><a href="#">0</a></span>
        </div>
      </div>
    </div>
    <div class="header-bottom">
      <div class="inner">
        <nav class="menu">
          <ul>
            <li><a href="#">Бытовая техника</a></li>
            <li><a href="#">Электроника</a></li>
            <li><a href="#">Строительство и ремонт</a></li>
            <li><a href="#">Дача и сад</a></li>
            <li><a href="#">Красота и спорт</a></li>
            <li><a href="#">Офис</a></li>
          </ul>
        </nav>
        <div class="sales">
          <a href="#"><span><img src="images/advantage-mini5.png">Акции</span></a>
        </div>
      </div>
    </div>
  </header>
  <div class="middle">
    <div class="center-image">
      <img src="images/slider1.jpg">
    </div>
    <div class="inner">
      <div class="sale">
        <div class="sale-top">
          <span class="sale1">Скидки</span>
          <span class="sale2">Уцененные товары</span>
        </div>
        <div class="gadgets">
          <div class="item">
            <div class="item_img">
              <img src="images/examples/product-item1.jpg">
            </div>
            <div class="item_stars">
              <img src="images/stars5.png">
            </div>
            <div class="item_text1">
              <span>Смартфон Apple iPhone 7</span>
            </div>
            <div class="item_text2">
              <span>32GB Black edition</span>
            </div>
            <div class="item_price">
              <span class="first-price">1500<sup>.00р.</sup></span><br>
              <span class="second-price">1939.00р</span>
            </div>
            <div class="icons">
              <span class="icon1"></span>
              <span class="icon2"><img src="images/cart-active.png">В корзине</span>
              <span class="icon3"></span>
            </div>
          </div>
          <div class="item">
            <div class="item_img">
              <img src="images/examples/product-item2.jpg">
            </div>
            <div class="item_stars">
              <img src="images/stars5.png">
            </div>
            <div class="item_text1">
              <span>Смартфон Apple iPhone 7</span>
            </div>
            <div class="item_text2">
              <span>32GB Black edition</span>
            </div>
            <div class="item_price">
              <span class="first-price">1500<sup>.00р.</sup></span><br>
              <span class="second-price">19390.00р</span>
            </div>
            <div class="icons">
              <span class="icon1"></span>
              <span class="icon22"><img src="images/cart.png">Купить</span>
              <span class="icon3"></span>
            </div>
          </div>
          <div class="item">
            <div class="item_img">
              <img src="images/examples/product-item5.jpg">
            </div>
            <div class="item_stars">
              <img src="images/stars5.png">
            </div>
            <div class="item_text1">
              <span>Смартфон Apple iPhone 7</span>
            </div>
            <div class="item_text2">
              <span>32GB Black edition</span>
            </div>
            <div class="item_price">
              <span class="first-price">1500<sup>.00р.</sup></span><br>
              <span class="second-price">19390.00р</span>
            </div>
            <div class="icons">
              <span class="icon1"></span>
              <span class="icon22"><img src="images/cart.png">Купить</span>
              <span class="icon3"></span>
            </div>
          </div>
          <div class="item">
            <div class="item_img">
              <img src="images/examples/product-item4.jpg">
            </div>
            <div class="item_stars">
              <img src="images/stars5.png">
            </div>
            <div class="item_text1">
              <span>Смартфон Apple iPhone 7</span>
            </div>
            <div class="item_text2">
              <span>32GB Black edition</span>
            </div>
            <div class="item_price">
              <span class="first-price">1500<sup>.00р.</sup></span><br>
              <span class="second-price">19390.00р</span>
            </div>
            <div class="icons">
              <span class="icon1"></span>
              <span class="icon22"><img src="images/cart.png">Купить</span>
              <span class="icon3"></span>
            </div>
          </div>
        </div>
      </div>
      <div class="banner">
        <img src="images/examples/banner.jpg">
      </div>
      <div class="sale-top">
        <span class="sale1">Хиты продаж</span>

      </div>
      <div class="devices">
        <div class="item">
          <div class="item_img">
            <img src="images/examples/product-item1.jpg">
          </div>
          <div class="item_stars">
            <img src="images/stars5.png">
          </div>
          <div class="item_text1">
            <span>Смартфон Apple iPhone 7</span>
          </div>
          <div class="item_text2">
            <span>32GB Black edition</span>
          </div>
          <div class="item_price">
            <span class="first-price">1500<sup>.00р.</sup></span><br>
            <span class="second-price">19390.00р</span>
          </div>
          <div class="icons">
            <span class="icon1"></span>
            <span class="icon2"><img src="images/cart.png">Купить</span>
            <span class="icon3"></span>
          </div>
        </div>
        <div class="item">
          <div class="item_img">
            <img src="images/examples/product-item2.jpg">
          </div>
          <div class="item_stars">
            <img src="images/stars5.png">
          </div>
          <div class="item_text1">
            <span>Смартфон Apple iPhone 7</span>
          </div>
          <div class="item_text2">
            <span>32GB Black edition</span>
          </div>
          <div class="item_price">
            <span class="first-price">1500<sup>.00р.</sup></span><br>
            <span class="second-price">19390.00р</span>
          </div>
          <div class="icons">
            <span class="icon1"></span>
            <span class="icon2"><img src="images/cart.png">Купить</span>
            <span class="icon3"></span>
          </div>
        </div>
        <div class="item">
          <div class="item_img">
            <img src="images/examples/product-item5.jpg">
          </div>
          <div class="item_stars">
            <img src="images/stars5.png">
          </div>
          <div class="item_text1">
            <span>Смартфон Apple iPhone 7</span>
          </div>
          <div class="item_text2">
            <span>32GB Black edition</span>
          </div>
          <div class="item_price">
            <span class="first-price">1500<sup>.00р.</sup></span><br>
            <span class="second-price">19390.00р</span>
          </div>
          <div class="icons">
            <span class="icon1"></span>
            <span class="icon2"><img src="images/cart.png">Купить</span>
            <span class="icon3"></span>
          </div>
        </div>
        <div class="item">
          <div class="item_img">
            <img src="images/examples/product-item4.jpg">
          </div>
          <div class="item_stars">
            <img src="images/stars5.png">
          </div>
          <div class="item_text1">
            <span>Смартфон Apple iPhone 7</span>
          </div>
          <div class="item_text2">
            <span>32GB Black edition</span>
          </div>
          <div class="item_price">
            <span class="first-price">1500<sup>.00р.</sup></span><br>
            <span class="second-price">19390.00р</span>
          </div>
          <div class="icons">
            <span class="icon1"></span>
            <span class="icon2"><img src="images/cart.png">Купить</span>
            <span class="icon3"></span>
          </div>
        </div>
      </div>
      <div class="about">
        <ul>
          <li>123</li>
          <li>123</li>
          <li>123</li>
          <li>123</li>
          <li>123</li>
        </ul>
      </div>
    </div>
  </div>

A:

у Вас body: font-size:0; замените на font-size:18px; 
что бы убрать расстояние между inline элементами достаточно просто закоментировать пробелы между ними вот так :

li{
display:inline-block;
padding:10px;
background:lightblue;
list-style:none;
}
<li>1</li><!--
--><li>2</li><!--
--><li>3</li><!--
--><li>4</li><!--
--><li>5</li>



второй вариант, написать в линию все inline блоки (спасибо @Herrgott)

li{
list-style:none;
padding:10px;
background:lightblue;
display:inline-block;
}
<li>1</li><li>2</li><li>3</li><li>4</li><li>5</li>

