About float and collapse

Question

The video talks about float. It says when a floated tag is inside a tag without float, it will collapse.

In the code, we set the outer tag white background color.

Firstly, I don't understand why this happen. Are there any general rules for us when we use float? For example, if we set inside tag float, we must also set outer tag float?

Second, the video says add float to the outer tag "main-nav" can solve this problem. Indeed, it works. However, according to his explanation, if I remove the float for inner tag, it should also be fine, right? Actually, it is not. Please refer to my code below. The white space "main-nav" is still empty!

/* Page Styles
================================ */

* {
    box-sizing: border-box;
}
html,
body{
    height: 100%;
}
body {
    font: normal 1.1em/1.5 sans-serif;
    color: #222;
    background-color: #edeff0;
}



/* Layout Element Colors
================================ */

.main-header       { background-color: #384047; }
.main-logo a       { background-color: #5fcf80; }
.main-nav a       { background-color: #3f8abf; }
.primary-content   { background-color: #caebf6; }
.secondary-content { background-color: #bfe3d0; }
.main-footer       { background-color: #b7c0c7; }

/* Main Layout Styles
================================ */

.main-wrapper {
    width: 90%;
    margin: auto;
}
.main-header {
    min-height: 100px;
    padding: 15px;
}

.main-logo {
    float: left;
    margin: 0 50px 0 0;
}
.main-nav {
    margin: 10px 5px;
    background: white;
    padding: 5px;
}
.main-nav li{

    margin-right: 10px;
}


.main-logo a, 
.main-nav a {
    display: block;
    color: white;
    text-decoration: none;
    text-align: center;
    padding: 5px 15px;
    border-radius: 5px;

}

.main-nav {

    display: inline
}
.main-banner {
    background: #dfe2e4;
    text-align: center;
    padding: 35px 15px;
}
.main-footer {
    text-align: center;
    padding-top: 5px;
    padding-bottom: 5px;
}

/* Column Layout
================================ */

.col {
    padding: 20px;
}

/* Media Queries
================================ */

@media (max-width: 768px) {
    .main-wrapper,
    .main-nav li, 
    .main-logo {
        width: initial;
        height: initial;
    }
    .main-logo {
        margin-right: 0;
    }
    .extra-content {
        display: none;
    }
}

<!DOCTYPE html>
<html>
<head>
    <title>Floats</title>
    <meta name="viewport" content="width=device-width">
    <link rel="stylesheet" href="css/normalize.css">
    <link rel="stylesheet" href="css/style.css">
</head>
<body>
    <div class="main-wrapper">
        <header class="main-header">
            <h1 class="main-logo"><a href="#">Logo</a></h1>
            <ul class="main-nav">
                <li><a href="#">Link 1</a></li>
                <li><a href="#">Link 2</a></li>
                <li><a href="#">Link 3</a></li>
                <li><a href="#">Link 4</a></li>
            </ul>
        </header>
<!--        <div class="main-banner">
            <h1>This is the Main Banner Heading</h1>
            <p>Andouille pork chop pancetta drumstick ground round beef ribs swine brisket ham.</p>
        </div> -->
        <div class="content-row">
            <div class="primary-content col">
                <h2>Primary Content</h2>
                <p>Bacon ipsum dolor sit amet chicken pork ground round brisket corned beef ball tip shank tail salami filet mignon ham hock pork belly venison shankle. Pig kielbasa drumstick sausage pork chop boudin. Chicken t-bone salami pork chop, beef ribs kevin ham tri-tip beef venison biltong brisket.</p>
                <p>Venison strip steak meatball chicken, brisket prosciutto sirloin. Capicola drumstick brisket tri-tip salami. Chicken beef jerky, tail turkey prosciutto cow ham sirloin boudin tenderloin. Meatloaf tri-tip turducken brisket andouille, pork belly corned beef fatback hamburger.</p>
            </div>
            <div class="extra-content col">
                <h3>Extra Content</h3>
                <p>Strip steak tenderloin kevin swine meatloaf capicola, doner beef turducken pancetta corned beef pork loin shoulder.</p>
                <hr>
                <p>Pork filet mignon leberkas, tail swine venison pancetta turkey shoulder brisket chalkers likes hamburgers.</p>
            </div>
            <div class="secondary-content col">
                <h3>Secondary Content</h3>
                <p>Strip steak tenderloin kevin swine meatloaf capicola, doner beef turducken pancetta corned beef pork loin shoulder.</p>
                <hr>
                <p>Pork filet mignon leberkas, tail swine venison pancetta turkey shoulder brisket chalkers likes hamburgers.</p>
            </div>
        </div>
        <footer class="main-footer">
            <p>&copy;2014 Example Layout</p>
        </footer>
    </div>
</body>
</html>

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