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HTML 

              
                <div class="wrapper">

<h1>Отдельный блочный контекст форматирования</h1>

  <p>Разные способы создания. Во всех примерах высокий желтоватый float стоит <em>перед</em> блоком, голубой квадратный float — внутри него, а треугольник демонстриует пример элемента, выступающего за край блока. Розовым подсвечены margin-ы дочернего элемента &lt;p&gt;. При наведении на желтоватый float можно увидеть, что будет, если в блоке окажется слишком широкая картинка или таблица.</p>

<h2>0. Обычный блок без БКФ (для сравнения)</h2>

<div class="outer-float"></div>

<div class="container">
  <div class="item"></div>
  <p>Обычный блок</p>
</div>
  
 <p>Проницаем для плавающих блоков, внутренних и внешних. Блок охватывает только неплавающий контент (текст), и то margin-ы абзаца (подсвечены розовым) выпадают наружу. И обычный clearfix решает лишь часть проблем:</p>
  
<h2>0a. Обычный блок с clearfix (тоже не создает БКФ)</h2>

<div class="outer-float"></div>

<div class="container clearfix">
  <div class="item"></div>
  <p>Обычный блок c clearfix</p>
</div>
  
<p>Псевдоэлемент с clear «зацепился» за внешний float и растянул контейнер больше, чем нужно. А margin всё равно вываливается.</p>

<h2>1. Стандартное решение с display: flow-root</h2>

<div class="outer-float"></div>

<div class="container container4">
  <div class="item"></div>
  <p>Блок с display: flow-root. Работает в Firefox 53+ и Chrome 57+.</p>
</div>

<h2>2. Особый случай: &lt;fieldset&gt;</h2>

<div class="outer-float"></div>

<fieldset class="container container4">
  <div class="item"></div>
  <p>&lt;fieldset&gt;, который всегда создает БКФ.</p>
</fieldset>

<h2>3. Хак с overflow</h2>

<div class="outer-float"></div>

<div class="container container3">
  <div class="item"></div>
  <p>Блок с overflow: hidden</p>
</div>

<hr class="clear">

<div class="outer-float"></div>

<div class="container container3">
  <div class="item"></div>
  <p>Блок с overflow: auto</p>
</div>

<h2>4. Хак с display: table</h2>

<div class="outer-float"></div>

<div class="container container5">
  <div class="item"></div>
  <p>Блок с display: table, в котором много текста. Достаточно много текста.</p>
</div>

<hr class="clear">

<div class="outer-float"></div>

<div class="container container5">
  <div class="item"></div>
  <p>А тут мало.</p>
</div>
  
<h2>5. Хак с display: table-cell</h2>

<div class="outer-float"></div>

<div class="container container6">
  <div class="item"></div>
  <p>Блок с display: table-cell, в котором много текста. Достаточно много.</p>
</div>

<hr class="clear">

<div class="outer-float"></div>

<div class="container container6">
  <div class="item"></div>
  <p>А тут мало.</p>
</div>
  
<h2>6. Хак с display: inline-block</h2>

<div class="outer-float"></div>

<div class="container container7">
  <div class="item"></div>
  <p>Блок с display: inline-block, в котором много текста. Достаточно много.</p>
</div>

<hr class="clear">

<div class="outer-float"></div>

<div class="container container7">
  <div class="item"></div>
  <p>А тут мало.</p>
</div>

Не забывайте, что инлайн-блоки живут в инлайновом контексте форматирования!

<h2>7. Хак с float</h2>

<div class="outer-float"></div>

<div class="container container8">
  <div class="item"></div>
  <p>Плавающий блок, в котором много текста. Достаточно много.</p>
</div>

<hr class="clear">

<div class="outer-float"></div>

<div class="container container8">
  <div class="item"></div>
  <p>А тут мало.</p>
</div>

Сами плавающие блоки тоже создают БКФ. Но какой ценой!

<h2>8. Хак с колонкой</h2>

<div class="outer-float"></div>

<div class="container container9">
  <div class="item"></div>
  <p>Блок с column-count: 1. В Chrome треугольник раньше обрезался из-за opacity...</p>
</div>

<hr class="clear">

<div class="outer-float"></div>

<div class="container container9 chromefix">
  <div class="item"></div>
  <p>...но это исправлялось через will-change или backface-visibility</p>
</div>

<hr class="clear">

<div class="outer-float"></div>

<div class="container container11 chromefix">
  <div class="item"></div>
  <p>Блок с <code>column-span: all</code>.</p>
</div>
  
<p>По текущей версии спецификации колонок, <code>column-span: all</code> создает БКФ даже без колоночного контейнера. Но пока только <a href="https://bugs.chromium.org/p/chromium/issues/detail?id=709362">в Chrome</a> и Edge 16+.</p>


<h2>9. Способ с изоляцией</h2>

<div class="outer-float"></div>

<div class="container container10">
  <div class="item"></div>
  <p>Блок с contain: layout;</p>
</div>

<p>Тоже практически стандартный способ. Работает в Chrome 52+ и аналогах.</p>
</div>
!

CSS 

              
                .clearfix::after {
  content: '';
  display: block;
  clear: both;
}

.container3 {
  overflow: hidden;
}
.container3 ~ .container3 {
  overflow: auto;
} 

.container4 {
  display: flow-root;
}

.container5 {
  display: table;
}

.container6 {
  display: table-cell;
}

.container7 {
  display: inline-block;
}

.container8 {
  float: left;
}

.container9 {
  -webkit-column-count: 1;
  -moz-column-count: 1;
  column-count: 1;
}

.container10 {
  contain: layout;
}

.container11 {
  -webkit-column-span: all;
  -moz-column-span: all;
  column-span: all;
}

.container {
  box-shadow: 0 0 0 2px #3bc9db;
  border-radius: 5px;
  background-color: #e3fafc;
  padding: 0 5px;
}

.container::before {
  content: '';
  float: right;
  border-style: solid;
  border-color: transparent #1098ad;
  border-width: 2em 0 2em 4em;
  margin: 1em -3em 0 0;
  opacity: .7;
}

.item {
  height: 100px;
  width: 100px;
  background-color: #1098ad;
  border: 1px solid #0b7285;
  border-radius: 5px;
  float: left;
  opacity: .7
}

h2 {
  padding: 1em 0 .5em 0;
  margin: 0;
  clear: both;
}

body {
  font: 18px Helvetica, sans-serif;
  background: #eee;
}

.wrapper {
  max-width: 500px;
  margin: 40px auto;
  padding: 5px;
  background: #fff;
}

.outer-float {
  height: 150px;
  width: 50px;
  background-color: #98ad10;
  border: 1px solid #72850b;
  border-radius: 5px;
  float: left;
  margin-right: 5px;
  opacity: .6
}

.container p {
  margin: 1em 0;
  box-shadow: 0 -1em 0 0 rgba(255,0,0,.15), 0 1em 0 0 rgba(255,0,0,.15);
  border-radius: 5px;
}
.container p::after {
  content: ' Пример широкой картинки в контенте ';
  display: inline-block;
  background: #dddd33;
  padding: 0;
  width: 0;
  white-space: nowrap;
  overflow: hidden;
  transition: width .4s;
}
:hover + .container p::after {
  width: 650px;
}

fieldset {
  -webkit-appearance: none;
  -moz-appearance: none;
  appearance: none;
  border: none;
}
hr {
  visibility: hidden;
}
.clear {
  clear: both;
}

.chromefix::before {
  will-change: opacity;
  /* так тоже работает:
  backface-visibility: hidden; */
}
!

JS
!
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