I’ll move this over to the CSS forum for now, to see if anyone there can help.
Yes, the inside white (besides the small curvy border between black and red) has to be transparent, the rest is pasted as a rectangle covering the page-background and the horizontal line.
If it is a dynamic level meter, then the img is in the foreground, and has a white background color.
Now you can use a red background image and move that dynamically up and down with the background-position.
So it is an image with a background image!
(Something [U]like this[/U] in “The port-hole theory: positioning of background-images in css”)