前言中的这段话非常核心：

The heart of the matter is that as web designers, we never know exactly how the pages we create will be viewed. We don’t know which of the dozens of browsers might be used, whether it is on a desktop computer or something more portable, how large the browser window will be, what fonts are installed, whether functionality such as JavaScript is enabled, how fast the internet connection is, whether the pages are being read by a screen reader, and so on.

这节主要阐述这些概念：

• The multitude of devices
• Web standards
• Progressive enhancement
• Responsive Web Design
• Accessibility
• Site performance

各个话题都提供了非常好的工具和扩展阅读。