One of many CSS tricks with calc:

.side {
    width: 33%;
    width: calc( 249px + 1px );
}
.main {
    width: 67%; 
    width: calc( 100% - 250px );
 }

A sweet little primer on CSS calc() function. From @MikeRiethmuller at #respond17 pic.twitter.com/TV1S9k1KYm

— Laura Summers (@summerscope) May 8, 2017