Issue link: https://read.uberflip.com/i/1146286
106 | Chapter 15: Deconstructing 10 Top Sites I used Firebug (http://www.getfirebug.com) to analyze the JavaScript and CSS in the various pages. Firebug is a critical tool for any frontend engineer. Its strongest fea- ture is the ability to debug JavaScript code, but that's just a fraction of what it can do. Firebug also provides functionality to inspect the DOM, tweak CSS, execute JavaScript, and explore the page's HTML. The main tool used to analyze the performance of these pages was YSlow (http:// developer.yahoo.com/yslow). I built YSlow for Yahoo! development teams to help them identify the changes that could lead to the greatest improvements in perfor- mance. Joe Hewitt, Firebug's author, provided support for the integration of YSlow with Firebug. This is an ideal combination since frontend engineers already use Fire- bug during development. YSlow crawls the page's DOM to find all the components in the page. It uses XMLHttpRequest to find the response time and size of each component, as well as the HTTP response headers. This, along with other information gathered from parsing the page's HTML is used to score each rule, as shown in Figure 15-3. The overall YSlow grade is a weighted average of each rule's score. YSlow provides other tools as well, including a summary of the page's components and an analysis of all the Java- Script in the page using JSLint (http://jslint.com). Figure 15-3. YSlow