Figuring it was about time to make a new one of these, I did. Since this is to be more advanced, instead of supplying the generic profile objects (such as .bio) in this site, I shall use the word selector (mostly because there are more specific selectors, such as img[alt=logo]) and, instead of the word code, declaration.
As always, the basic syntax is:

[style]selector{declaration}[style]

Position

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This is one of those declarations that should not be overused, but it can be used on any selectors.

{position:fixed;}

This declaration causes a selector's position, relative to the browser window, to become fixed, so that the selector will move along with the scrolling of the screen. For some reason, this declaration does not work in my 3DS browser, though.

A selector's position, relative to the browser window, can be affected using the following declarations:
• {top:numberpx;}
(determines the number of pixels the selector is from the top of the browser window)
• {left:numberpx}
(determines the number of pixels the selector is from the left of the browser window)
• {right:numberpx}
(determines the number of pixels the selector is from the right of the browser window)
• {bottom:numberpx}
(determines the number of pixels the selector is from the bottom of the browser window)

Those declarations and the position declaration can combine well for a better appearance.

Opacity

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Another declaration that should not be overused but can be implemented on any selector.

{opacity:number}

The number can be between 1.0 (completely opaque) and 0.0 (completely transparent).

Visibility

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Overusing this declaration could get you banned, so yeah...

{visibility:hidden;}

This makes the selector hidden.

Saving Space

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We all want our profiles to look good, but the character limit for the DSi or 3DS can make that frustrating. Even without the character limit, all of that typing can be tiring. Fortunately, these methods should make it easier.

If you have multiple selectors that share the same declaration, you can combine the selectors, separating them with commas (,):

selector,selector{declaration}

If you have a selector with multiple declarations, you can combine the declarations, separating them with a semicolon (;):

selector{declaration;declaration}

If you have multiple selectors sharing multiple declarations, you can combine them using the above ways:

selector,selector{declaration;declaration}

A Final Note

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If you have any questions regarding this, this site should be able to provide answers. I hope I helped make your profile more interesting.