The basics of scripts and blocking
HTML content that appears after a tag will not display until the script is done downloading, parsing, and executing (blocking example)
tags prevent the document ready event from happening until they are done downloading, parsing, and executing
If these points make sense, skip forward to tracking scripts.
Script tags block
When a browser renders a web page it reads the HTML from top to bottom. When it encounters a tag—maybe in the , maybe in the —it will halt from rendering the rest of the page, i.e. nothing else will show up, until the script has finished downloading, parsing, and executing.
Later scripts get blocked since a script can modify the page, add additional scripts, or add new variables to the window object. Some modern browsers will download multiple scripts in parallel (ie8, safari4) and future versions of browsers are expected to do this too, but, for the reasons just mentioned, scripts will still block the execution of later scripts and page content that is after a script will not be displayed until the script has finished.
Capturing the page load event