CSS block-level vs inline container issue - html

Question

Here is the demo http://jsfiddle.net/aTBWh/
the container is id(div) meaning it inherits a display:block value from the browser, the two div's inside this container are classes. they both have 200px and 300px while the main container has 600px width, so I thought when I floated one of the classes to the right, it should only consume 300px from the whole container meaning the two div's should fit inside the container without one appearing above the other. there is no clear:both attribute.
if the container is an id, with 600px, then the classes nested inside it (specially when one is floated to right, they should fill the container.)
in a nutshell why is the green div class out of the container when it can clearly fit there, and it is floated to the right? I don't understand this problem.
codes:
css
#content_canvas_container{
background:#CCC;
min-height:200px;
border:1px solid red;
width:600px;
}
.red{
width:200px;
background:red;
height:140px;
}
.green{
width:300px;
background:green;
height:140px;
float:right;
}
/*PROPERTIES*/
.w90{width:98%}
.m_auto{margin:auto; border:1px solid black}
html
<section id='content_canvas_container'>
<div class='w90 m_auto'>
<div class='red'> red </div>
<div class='green'> green </div>
</div>
</section>

Answer 1

What you are seeing is expected behavior.
The reason this is occurring is because the red div element is a block level element by default. Thus, in its current state, it will always appear on a new line regardless of whether it has a defined width or has floating sibling elements.
By floating or absolutely positioning an element, you are essentially removing it from the flow of the document, rendering display:block ineffective. Therefore you can solve this by either floating the red div element, or adding display:inline-block to it. (example)
jsFiddle example
.red {
width: 200px;
background: red;
height: 140px;
float: left;
}

Answer 2

That happens because when you set a float to a box, it moves to the left or the right of the container (according with other complex rules, of course), but it's vertical offset from the top of the container can't be smaller than it would be if it wasn't floated.

Answer 3

The browser is making a decision based on what you are telling it to do.
You haven't specified a float on the red div, so it remains in flow and acts as a normal block level element does i.e. "push everything after me to the next row".
You then tell the green div to float right in it's container, so it shifts over to the right.

Answer 4

Using float:left will allow other elements to wrap around it. (CSS-float)
The float CSS property specifies that an element should be taken from the normal flow and placed along the left or right side of its container, where text and inline elements will wrap around it. A floating element is one where the computed value of float is not none.
.red{
float:left;
}
Fiddle
To understand why other elements won't wrap around your div by default, read up on block-level elements here.
"Block-level" is categorization of HTML elements, as contrasted with "inline" elements. Block-level elements may appear only within a element. Their most significant characteristic is that they typically are formatted with a line break before and after the element (thereby creating a stand-alone block of content). That is, they take up the width of their containers.
By default your div elements are block-level until you specify otherwise. The link I referrenced gives a list of all the elements that are block-level by default.

Answer 5

This happens because DIV are block-level elements, it means they begin in new lines. In your example when floating .green to the right, .red element still takes 100% of horizontal space, and .green takes now 300px but it is pushed down because as a block level element it belongs to the next line. To avoid this behavior, you must transform block elements into inline elements.
So in this case if you would like to float .green to the right, DIV elements within the parent wrapper should be rendered as inline blocks instead of independent blocks, just adding this rule:
.m_auto div {
display: inline-block;
}
Cheers.