what's the tag to make a background stretch across the whole screen instead of tiling?
thanks in advance

There's no code for that.... if you want your background to stretch, you'll have to do it in a graphics program. Then, of course, it may not be compatible for all resolutions.

ive seen a HTML code for that i was just wandering if there was a CSS code

What's the HTML code? You can stretch an image....

Well, you can stretch an image using HTML if you just adjust the height and width:
<img src="img.jpg" height=xx width=xx>

But there's no way to do that using css.