I love the way that Ire writes. In this article she goes into details about the process a browser goes through to render a web page. This is really important to understand as it gives you direct insight into the reasons behind some of our current implementation processes and also how they might change with the arrival of HTTP2.
When a browser receives the HTML response for a page from the server, there are a lot of steps to be taken before pixels are drawn on the screen. This sequence the browsers needs to run through for the initial paint of the page is called the “Critical Rendering Path”.An excerpt from Understanding the Critical Rendering Path