Question

How does the CSS Block Formatting Context work?

CSS2.1 specifications says that in a block formatting context, boxes are laid out vertically, starting at the top. This happens even if there are floated elements in the way, except if the block box established a new block formatting context. As we know, when browsers render block boxes in a block formatting context, the floated element is omitted, why does establishing a new block formatting context work?

How are boxes (block boxes and inline boxes) laid out in the normal flow?

I read somewhere that block elements generate block boxes, but floating elements are ignored when a user agent draws box and take them into account when they fill out content. Whilst floating elements will overlap other elements's boundary of the boxes, the solution is establishing a new block formatting context for the overlapped elements using overflow:hidden.

"New block formatting context is still block formatting", so when drawing a box, it will also treat the floating element as if it doesn't exit. Is that right or have I misunderstood "new block formatting context?"

Update:more questions

By saying "It's this behaviour that's useful for columnar style layouts. The main use of it however is to stop floats, say in a "main content" div, actually clearing floated side columns, i.e. floats that appear earlier in the source code."

I don't understand the meaning, I will provide an example, maybe you will understand me.

.content {
  background: #eee;
  color #000;
  border: 3px solid #444;
  width: 500px;
  height: 200px;
}
.float {
  background: rgba(0, 0, 255, 0.5);
  border: 1px solid #00f;
  width: 150px;
  height: 150px;
  float: right;
}
p {
  background: #444;
  color: #fff;
}

<div class="content">
  <h3>This is a content box</h3>
  <p>It contains a left floated box, you can see the actual content div does go under the float, but that it is the &lt;h3&gt; and &lt;p&gt; <b>line boxes</b> that are shortened to make room for the float. This is normal behaviour.</p>
  <div class="float">floated box</div>
</div>

Answer 1

Block Formatting Contexts

Floats, absolutely positioned elements, block containers (such as inline-blocks, table-cells, and table-captions) that are not block boxes, and block boxes with 'overflow' other than 'visible' (except when that value has been propagated to the viewport) establish new block formatting contexts for their contents.

With my bold, it's the establish bit that is important

What this means is that the element you use overflow (anything other than visible) or float or inline-block..etc on becomes responsible for the layout of its child elements. It's the child elements which are then "contained", whether that's floats or collapsing margins they should be wholly contained by their bounding parent.

In a block formatting context, each box's left outer edge touches the left edge of the containing block (for right-to-left formatting, right edges touch)

What the above line means:

Because a box can only be rectangular and not irregular shaped this means a new block formatting context between two floats (or even beside one) will not wrap around neighbouring side floats. The inner, child boxes can only extend as far as to touch their parents left (or right in RTL) edge. It's this behaviour that's useful for columnar style layouts. The main use of it however is to stop floats, say in a "main content" div, actually clearing floated side columns, i.e. floats that appear earlier in the source code.

---

Float Clearance

In normal circumstances floats are supposed to clear all previous floated elements, that's previously floated in the whole source code, not just your displayed "column" The telling quote from the "float clearance specs" is:

This property indicates which sides of an element's box(es) may not be adjacent to an earlier floating box. The 'clear' property does not consider floats inside the element itself or in other block formatting contexts

So say you have a three column layout where the left and right columns are floated left and right respectively, the side columns are now in new Block Formatting Contexts, because they are floated (remember float is also one of the properties that establish a new BFC), so you can happily float list elements inside them and they only clear floats which are already inside the side columns they no longer care about floats previously in the source code

---

To make the main content a new Block Formatting Context or not?

Now that middle column, you can simply margin it from both sides so that it appears to sit neatly between the two side floated columns and take the remaining width, a common way to get desired width if the centre column is "fluid" - which will be fine until you need to use floats/clearance inside your content div (a common occurrence for those using "clearfix" hacks or templates including them)

Take this very simple code:

#left-col {
  border: 1px solid #000;
  width: 180px;
  float: left;
}
#right-col {
  border: 1px solid #000;
  width: 180px;
  float: right;
  height: 200px;
}
#content {
  background: #eee;
  margin: 0 200px;
}
.floated {
  float: right;
  width: 180px;
  height: 100px;
  background: #dad;
}

<div id="left-col">left column</div>
<div id="right-col">right column</div>

<div id="content">
  <h3>Main Content</h3>
  <p>Lorem ipsum etc..</p>
  <div class="floated">this a floated box</div>
  <div class="floated">this a floated box</div>
</div>