I am newbie in web design. I have one rule that everything should be flexible. I don't understand why px are used widely in web design instead of percentage, maybe I am wrong.  

I have been looking for base html layout like on the picture below for a long time, but found only examples where some magic negative margins/paddings and PX are used  

I want to create such layout but using percentage without any px attribute. 

I am not sure if I am right , but I hate everything tightly dependent. All examples seems not flexible for me, of course I can use media queries, but I need to get such result using percentage or whatever not tight to the screen dimension. 

<https://cdn.tutsplus.com/net/uploads/legacy/543_html5/2_Column_Layout.png>

Please, can someone provide such example HTML + CSS (HTML 4 not 5).