学习笔记摘自freecodecamp.org提供的Responsive Web Design Certification(300 hours) 教程
All HTML elements are essentially little rectangles.
The main HTML5 tag helps search engines and other developers find the main content of your page.
Every HTML page has a body element.
you can style your body element just like any other HTML element, and all your other elements will inherit your body element's styles.
By creating your variables in :root, they will be available throughout the whole web page.