Why "Overflow: Hidden" Clears a Float

Why does 'overflow: auto' clear floats? And why are clear floats needed?

The reason why the wrapper doesn't stretch to contain its floats by default is because the floats are taken out of the normal flow of the wrapper, so the wrapper acts as if they were never there. If there is no other content in the wrapper, that means the wrapper won't have any height.

Note that overflow: auto doesn't clear floats — it just causes the element to contain its floats by way of establishing a new block formatting context for them so that they don't intrude to other elements in the parent context.¹ That is what forces the parent to stretch to contain them. You can only clear a float if you add a clearing element after the float. A parent element cannot clear its floating children.

The reason why establishing a new BFC causes an element to contain its floats is detailed here, and the reason why overflow: auto would even cause a BFC to be established is detailed here.

¹ _{OK, maybe "just" wasn't exactly the best adverb to use.}

Why does overflow hidden stop floating elements escaping their container?

It creates a block formatting context.

Block formatting contexts are important for the positioning (see float) and clearing (see clear) of floats. The rules for positioning
and clearing of floats apply only to things within the same block
formatting context. Floats do not affect the layout of things in other
block formatting contexts, and clear only clears past floats in the
same block formatting context.

see also: http://www.w3.org/TR/CSS2/visuren.html#block-formatting

CSS overflow:hidden with floats

I try to end the confusion:

ul is a block-level element as is the p element (they stretch to 100% of the parent width). That is why per default the p will appear below the ul if no width or display is declared on those elements.

Now in your example the ul contains only floated elements. This makes it collapse to a height of 0px (It still has 100% width though as you can see in the example). The adjacent p will appear to the right of the floated lis because they are considered as normal floated elements.

Now declaring overflow (any value other than visible) establishes a new block formatting context, which makes the ul contains its children. Suddenly the ul "reappears", not having size 0px anymore. The p is getting pushed to the bottom. You could also declare position:absolute to achieve the same "clearing" effect (with the side effect that now the ul is taken out of the normal element flow - the ps will be overlapped by the ul.)

See the example fiddle

If you are into the technical stuff, compare the according paragraphs of the CSS spec:

§10.6.3 Block-level non-replaced elements in normal flow when 'overflow' computes to 'visible'

and

§10.6.7 'Auto' heights for block formatting context roots. (Thanks to BoltClock for digging out the links).

ul{    list-style-type:none;    margin:0; padding:0;    background-color:#dddddd;    border:2px solid red;}li{    float:left;}a{    display:block;    width:60px;    background-color:#555;    color:white;}p{    margin:0;    outline:2px dotted blue;}#two{    clear:both;    overflow:hidden;}

No overflow:<ul><li><a href="#home">Home</a></li><li><a href="#news">News</a></li><li><a href="#contact">Contact</a></li><li><a href="#about">About</a></li></ul><p>Notice the collapsed ul - no background-color visible, collapsed border and this paragraph treats the lis as regular floats  </p><br>With overflow: hidden<ul id="two"><li><a href="#home">Home</a></li><li><a href="#news">News</a></li><li><a href="#contact">Contact</a></li><li><a href="#about">About</a></li></ul><p>the ul reappeared - it now contains the child li's - the float is cleared</p>

what is the difference between 'overflow and clear to clear float?

Strange comparison since overflow and clear are completely unrelated. Unless I misunderstood your question. In which case, please rephrase so that we can clarify better.

Anyhow, overflow controls the any excess outside of the width of an element.

The overflow property specifies what happens if content overflows an element's box.

If you have a div with containing a large image and you want to restrict the image to not exceed the width of that container, overflow will do just that by giving it a hidden value. If you want it to scroll after a certain width or height, the scroll value will activate the scrollbars to allow you to do so.

Clear on the other hand, resets the floats.

The clear property specifies on which sides of an element floating elements are not allowed to float.

This is particularly helpful in responsive design to center an item that has been floated to the right in larger displays but you want to reset it to the native left position for smaller devices. Of course, the use of clear can be determined by other factors according to your need of it.

The example above mentioned would look like this

<div class="box">
  <button class="right">Hello</button>
</div>

CSS

.right{
   float: right;
}
@media (max-width: 420px){
   .right{
       clear: right;
   }
}

Overflow:hidden in float. Does it hide anything?

As its name suggests, overflow:hidden hides any content that overflows the element (i.e. excesses the given dimensions). In this example there is no overflow actually, so nothing is hidden. But the side-effect of setting the overflow other than visible (as well as display: inline-block, float itself etc.) is the change of the behavior of the block. While the normal block effectively doesn't take floats into account at all (only its text content does), the block that establishes new block formatting context (see Adrift's answer) isolates all of its content inside, including nested floats, potentially collapsible margins etc.

This side effect may be used to prevent the container of floats from collapsing, but it doesn't have anything to do with clearing floats. Clearing and BFCs act very differently in case of collapsible margins, other floats earlier in the document, etc.

how does the overflow:hidden; works with float

overflow: hidden; only has a visible effect if you define width and height for that element and its contents would normally go beyond that width and height:

(widthhas a default of 100%, so it doesn't necessarily have to be defined in all situations)

.parent{
    background-color: red;
    padding: 10px;
    overflow: hidden;
    width: 200px;
    height: 15px;    
}
.parent div{
    background-color: #eee;
    float: left;

}

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" href="css/float.css">
    <title>Document</title>
</head>
<body>
    <div class="parent">
        <div>product one</div>
        <div>product one</div>
        <div>product one</div>
        <div>product one</div>
    </div>
    <p>this is a paragraph</p>
</body>
</html>

Do clear:both and overflow:hidden work the same way to make a container wrap floated children?

Do clear:both and overflow:hidden work the same way to make a container wrap floated children?

No. They perform different functions.

clear:both

The clear property controls whether an element can be next to or below floated elements that come before it. It controls whether an element can clear the floated elements.

clear:both, when applied to a non-floating, block-level element:

Requires that the top border edge of the box be below the bottom outer edge of any right-floating and left-floating boxes that resulted from elements earlier in the source document.

_{source: https://www.w3.org/TR/CSS2/visuren.html#propdef-clear}

So the clear property more likely applies to a sibling of floated elements. It has nothing to do with stretching the height of div which has float children (as stated in your question).

overflow:hidden (or overflow:auto)

The overflow property, when used with a value other than visible, creates a new block formatting context. This causes the element containing floats to expand in order to contain its floated children.

In summary, one property clears an element past floated elements. The other stretches the container to wrap floated elements. The output may appear the same for both. But each property is fundamentally different.

clear both or overflow hidden , a clearfix solution

overflow:hidden makes the element establish a new block formatting context. This fixes the float containment of any children floating within it. This CSS fix is more practical then including an additional element in the HTML styled with clear:both and works on all modern browsers, including IE7+.