Confusion about negative margin of float none and float left elements - html

Question

I feel like CSS is much harder and confusing than C++ therefore I have few questions.
Consider following html body
<div id="mydiv1">12345~~~~~~~~/</div><div id="mydiv2">+_______67890</div>
And CSS
#mydiv1 {
float: left;
background-color: red;
margin-right: -30px;
}
#mydiv2 {
float: left;
background-color: blue;
}
which looks like this (in my latest Chrome)
which makes sense to me because second div is floating and it floats over first div.
On the other hand if I remove float property from mydiv2 only content moves but background box stays in the same place.
1) Could you please explain why ?
Now I'll remove margin and float, and add width to both divs making having CSS
#mydiv1 {
background-color: red;
width: 220px;
}
#mydiv2 {
background-color: blue;
width: 240px;
}
It will expectantly look like this
But if I add float: left to #mydiv1 it suddenly looks like this
2) Why did second div become twice as high ? I checked it by setting z-index of first div to -1.
PS. I've done tutorials on CodeAcademy and read float/margin-related articles on smashingmagazine.com. Sadly it didn't made everything crystal clear. If you guys can suggest online resources or book that would have explained these questions to me I'll appreciate it a lot.

Answer 1

<div> is a block-level element so it naturally fills the width of the container it's in. It makes its neighboring elements go above/below it, but not beside it.
Now, when you apply float to a block-level element, it no longer fills the width of the container, its width will be that of its contents. It also loses the ability to force its neighbors to go above/below it.
Note:The tricky bit is that the container holding the floated elements will not have a proper height because the floated elements are no longer part of the regular flow of content. (Here's how to get around it: http://www.quirksmode.org/css/clearing.html)
Regarding the last part of your question, if a floated element, eg. #mydiv1, is beside a block-level, eg. #mydiv2, then the block-level element wraps or flows around the floated element. It's one of the ways people can get text to wrap around an image in a news article.

Answer 2

When you remove the float from div2 it goes behind the floated div1, because floated elements does not take any height from it's content. You can say it's going out of the vertical flow of elements. However, it still take horizontal space from content. So the result is as expected here, once you "know the rules".
This should also explain the double height in your other example.
Here is a great article from css-tricks.com
I hope that helps!

Answer 3

If we don't give either float or width to any block level element like div then it occupies the entire width of the container.
Instead of float you can give some width and display: inline-block. This display property will display content inline and behaves like a block level element.