This DIV's width is 50% of the containing block. The containing block is 601px wide. On a device with 1 pixel = 1px, how wide is this DIV? If you see a 1px vertical orange line, it is because the UA rounded 300,5 down to 300px. If you see a horizontal scrolling mechanism, the UA rounded it up to 301px. If you see no orange line, you may have a 200dpi screen. If you see a thicker line, something is wrong…