3. Find the right padding.
Because of the transformation, some elements bleed into the previous and the next element. To find a safe area where you can place content, you need to add some padding. The amount of padding can be calculated with this formula:
x = tan(α) * a / 2
Sadly you can not make this completely dynamic without javascript, as CSS calculations don't support sin, cos and tan.
Pro Tip: I think most of you will use deg as unit, when you do the transformation: skewY(-11deg).
If you do so, you also have to use Deg and not Rad when you calculate tangens. The standard uses Rad as default.