CSS is funny and often it seems like there is no reason for your problem...that said, I have found that (in my case) it's usually a careless error. To make your job easier try consolidating your code. For example take this code:

.tab {
background-color: #0033FF;
width: 100px;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
font-style: normal;
font-weight: bold;
text-transform: capitalize;
border-top-width: thin;
border-right-width: thin;
border-bottom-width: thin;
border-left-width: thin;
border-top-style: solid;
border-right-style: solid;
border-bottom-style: solid;
border-left-style: solid;
border-top-color: #000000;
border-right-color: #000000;
border-bottom-color: #000000;
border-left-color: #000000;
}

And turn change it to this:

.tab {
background-color: #0033FF;
width: 100px;
font-family: Arial, Helvetica, sans-serif;
font-size: 12px;
font-style: normal;
font-weight: bold;
text-transform: capitalize;
border: thin solid #000;
}

This will do exactly what the original code did with 1/3 of the code, bandwidth, and headaches

I know that didn't exactly answer your question but sometimes you just have to make your code a simple as possible...to avoid errors and to more easily debug those errros that occur