In the em series of test, increasing values from 0 to 100em with a step of 0.1em are given to a certain property. The correct result should be a straight black diagonal line or a solid black triangle. Presence of steps indicates rounding or quantization errors (for example because the fractional part of the value is ignored.) Resizing the browser text size should resize accordingly the whole rendering.
In IE/Win the resizing of borders expressed in em is not correct, unless there is at least a font-size expressed in percentage in an ancestor element. See IE/Win incorrect resizing of borders in em units, when the default font-size is changed.