Background attachment testcase
The four available stylesheets for this page place the stripes
along the four edges of the page. My understanding of CSS
(which may be wrong) is that "background-attachment: scroll;",
applied to the <body> element should place the background
relative to the containing block for the element.
The containing block for <body>, IMHO, is the area occupied by
the rendered content. In the case that your viewport (window)
is smaller than this area, that means the area around which
you can scroll.
In the case that your viewport is larger than your rendered
content, a background at the edge of the content will not
necessarily be at the edge of the window.
Positioning the background at the top or left works fine;
there can be no dispute about where that is. Positioning
at the bottom seems to be OK too - if your window is taller
than the content, the stripes will be at the end of the text;
if your window is shorter, the stripes scroll with the text.
However, Mozilla seems to think horizontal positioning is
calculated as an offset from the left side of a rectangle
which is as wide as the viewport, but has the origin of the
rendered content.
So if you put the stripes on the right and your window is
wider than this text, the stripes will be at the edge of
the window, not the edge of the text. This is the opposite
of the way positioning at the bottom works.
Similarly, if you make your window narrower than this text,
and scroll right (ie move the text to the left, then some of
the content will appear to the right of the background. In
this case, the background, which is supposed to be positioned
right, is neither on the right-hand edge of the content, nor
the right edge of the window.
Hence I think it's a bug. But I may be wrong.