CSS
The underlying principle of CSS is quite simple even though it is capable of so much.
It stands for Cascading Style Sheets - external document(s) that store all the design aspects of your page ie heading attributes, fonts, colours, link behaviours, background images and even layout to name but a few.
By storing these as a separate file your web browser only needs to load this information once - making your site much more efficient. Another advantage is being able to design pages without the use of tables - making your code much lighter and faster.