I have the following CSS and HTML snippet being rendered.
The answer to many CSS formatting problems seems to be "add another
So, in that spirit, have you tried adding a wrapper div to which the border/padding are applied and then putting the 100% width textarea inside of that? Something like (untested):
textarea
{
width:100%;
}
.textwrapper
{
border:1px solid #999999;
margin:5px 0;
padding:3px;
}