Which unit I should use in CSS when designing a web page

Question

I have designed and developed more than 10 sites, but I still have a doubt about the correct unit I should use. Should it be px, em or %

Answer 1

There's no real right or wrong, but as a rule of thumb:

• For anything you want a certain, fixed size, use PX
• For anything you want to scale with font-size, use EM
• For anything you want to scale to the available space in the window/container, use %

Each used to have specific advantages or disadvantages in different browsers when it came to users scaling the browser's base font-size/zooming, but more recent versions of the browsers by-and-large get around these issues by scaling everything, not just font-size.