I am learning about Relative Unit of measurement for responsive design. But i am little bit confused seeing many designers are using em unit in defining their container width. I have also seen while defining media queries,the breaking point is defined in em. what is benefit of using em in layout. I will be thankful to you who can show me real life benefit of em in using in layout. I used em in layout of my design. i was expecting that em will behave like percentage while squeezing browser window but it behaved like pixel. so, why does designers prefer em instead of px ?