Margin-bottom disappears inside a div

Maybe it has been posted thousand of times, but I’m not able to find the answer nor in Google neither in SitePoint forum…
The question is pretty easy: why, in the below page, the margin-bottom of the paragraph is ignored? I’d expect it would “expand” the parent div, but it ain’t so:

Is it a “clearing” problem? I don’t think, since there are no floats: anyway, adding classic “clearfix” rule to the parent div solves the problem in all browsers but Chrome
Is it a “margin collapse” issue? What margin are exaclty collapsing? I know that margin collapse when elements are one on top of eachother, but here I can’t see “elements above and below”
Using “overflow:auto” on parent div works but… why???
Please, is there a definitive article that explains me what’s going on?
How is this (strange? normal?) behavior known out there?

Thanks in advance…

Thanks Gary, the article was very interesting!
I knew about Collapsing Margins Between Adjacent Elements but I was missing the Collapsing Margins Between Parent and Child Elements part :slight_smile:

You are experiencing margin collapse. See Sitepoint’s collapsing margin article.