border size and combining units

is there a reason that borders-widths cannot be percentage values?  it would 
make it that much easier to create fluid GUIs if they could.

also (and i believe that this was suggested a while ago, though i can't 
remember when or by whom) there should be some way to add two different 
units together to size an element:

elm1 {
    position: absolute;
    left: 0%;
    width: 50%;

    margin: 10px;
}
elm2 {
    position: absolute;
    left: 50% + 20px;   /* to accomodate elm1 */
    width: 50% - 20px;
}

                                                  • mjumbe •

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