Author, developer, designer & photographer
Promises are great but the its nested syntax can get easily complex and hard to follow. With ES2017 async and await keywords for writing asynchronous code makes it more readable and easier to follow than equivalent code based on long promise chains or deeply nested callbacks.
Web page rendering is an important subject to study in order to optimize page load times, and to understand how to design solutions to cater for that. For understanding how web pages get rendered, we need to study how browsers render a web page.