Without dynamic properties, element properties must be declared with constants only. The problem with constant properties is that they are not suitable for animation.
Animation seems to be the complete opposite of constancy.
In order to implement an animation in an HTML page, you had to explicitly recompute element properties over and over again, every pre-defined time interval. You can find such animation all over our columns. For example, see Column 18, JavaScript Animations, Part I and Column 19, JavaScript Animations, Part II . Using dynamic properties, you would write much less code. You still need to specify in the beginning of your code the functional dependency of element properties, and then you let JavaScript do the job for you.