I want to achieve this using html and css:

I have tried to set the opacity of the container to 0.3 and the box to 1, but it doesn’t work: both divs have 0.3 opacity.
The effect I am trying to achive is a popup box that comes on top of the page. It is highlighted by fading the content below (by lowering the opacity).
As far as I know you can’t do it in a simple way. There a couple of options here:
Use absolute positioning to position box "inside" the container.
Use Javascript – almost the same as above, but position and size don’t have to be hardcoded.