The file extension has nothing to do with it. It could be .ASPX, .ASCX, .PHP, .HTML, .WTF, etc.
The "margin: 0 auto" does not center anything. It defines your top and bottom margins as zero while your left and right margins automatically scale according to their parent element.
If your content area (.inside) weren't a defined width (in pixels, that is), you could center it using margins. Setting the width to 50% and the left and right margins to 25% each will give you a centered div that's 50% the width of its parent element (.outside). But if you're assigning a pixel-width to the .inside div while the width .outside div varies, you won't know what value to assign the margins.
.outside {
width: 100%;
}
.inside {
width: 50%;
margin: 0 25%;
}
This works, but because your inside container width is set to a percentage of the outside container, the dimensions of the outside container will ultimately determine the width of its contents.