[jQuery] question about attr()

[Guapo]

i want to make all tds have a top vertical align, so i write the
following code:

$("td").attr("valign","top");

but it failed. and i found if i change the letter a in "valign" to
uppercase, it works well. code as follows:

$("td").attr("vAlign","top");

anyone can explain it?
test on window xp sp2, ie7. firefox works well all the time.