maintainable css. Starting with the smallest screen size that you want to support and changing the layout accordingly for the bigger sizes. Don’t fall prey to the notion of developing a responsive website at all costs. It sometimes makes sense to serve a separate website for bigger screens, with added functionality. Building a mobile first website helps you avoid the pain of porting a layout from a bigger screen to a smaller one.
After defining the layout, you add colors and icons to your design. You do this so you don’t share information solely through your styles. This is important as people have impairments that you aren’t aware of. Adding images should be the last step as they tend to be the last to be loaded. You should add relevant alt tags and display the images as css gradients while they are loading.