onload set width

[Mr. x]

Hello,
I am using the onload event.
by the event I call function : myLoad
on myLoad I put a code like this :

function myLoad()
{
mytable1.width = 100% - 10
mytable2.width = 100% - 10
}

the line mytable1.width = 100% - 10 is executed,
but the next line isn't execute at all - after the first line is executed,
the function returns to the caller.

Why I have the above problem, and how should I avoid the problem ?

Thanks