Application of Attributes

Text and Images from Slide

By creating a "style" in your CSS document that includes all aspects of the element you want displayed in a specific way, you allow the end-user to substitute their own styles based on need or preference.
An example are web pages created by people with colour deficits who create pages that are usable by them but may not be useable by anyone who does not have a similar colour deficit. Same with pages done in large print, smaller print and so forth.
The key to creating an accessible and usable web page is to make it possible for the end-user to choose how they view and interact with your content.
Can still have branding but allow different means of accessing the information.
Using percentages makes the content scalable which can help when viewed on mobile devices.