This is a good tip, and it makes sense. Because every element on the screen is laid out based on its size, if something on the screen changes, the size and placement of every element on the screen must be recomputed. At best. At worst, the entire screen is recomputed and re-rendered. Honestly, that's IE's job, so I'm not sure which of those it is doing. Either way, it's fairly intensive.
As an example, use a more proportional font (rather than a fixed-width font) for the clock. A font where a "1" takes up less horizontal space than a "0". As the clock ticks down, you'll see that the entire clock is redrawn every second, because sometimes it takes up more horizontal space than at other times, and because it is centered, it must be shifted left and right to keep it centered. Furthermore, when the clock is smaller (say after it counts below 10 minutes, when it goes from 10:00 to 9:00), the elements around it are given more space and thus are redrawn.
You, in effect, prevented IE from having to redraw (or at least recompute) the screen every second. I imagine that's pretty huge.