Why is the text visible, but the background is not?

[Nebez]

I have following mark up:\[code\]<div class="one">Content</div><div class="two"></div>\[/code\]and following style sheets:\[code\].one, .two { height: 20px;}.one { background-color: #f00;}.two { background-color: #0f0; margin-top: -10px;}\[/code\]Why is the text \[code\]Content\[/code\] visible, but the red background is not? I would expect the text also to be only partly visible due to the given style sheets.For your convenience, I also created a jsFiddle.