css problems

Ive gotta write this stuff down before i forget it all:

problem:
z-index does not work. div with a higher z-index is getting overlapped by a div with lower z-index

solution:
one of the divs need to have this set -> position:absolute;



problem:
i need IE and Firefox styles on the same style sheet using a hack

solution:
#someId {
/* IE can only see this rule, IE uses this rule */
position: relative;
top: -15px;
}
html>body #someId {
/* Firefox can see this rule, rule overrides previous rule */
position: relative;
top: 25px;
}



problem:
i want to change the position of a div, lift it up by 100 pixels

solution:
there are 2 ways to do this,
margin-top:-100px;
or
position:relative;
top:-100px;

these have slightly different results, and i should try both.