Here are the details from the Google Developers site:
CSS & JS Code Coverage
Find unused CSS and JS code with the new Coverage tab. When you load or run a page, the tab tells you how much code was used, versus how much was loaded. You can reduce the size of your pages by only shipping the code that you need.
Figure 1. The Coverage tab
Clicking on a URL reveals that file in the Sources panel with a breakdown of which lines of code executed.
Figure 2. A breakdown of code coverage in the Sources panel
Each line of code is color-coded:
- Solid green means that line of code executed.
- Solid red means it did not execute.
- A line of code that is both red and green, such as line 3 in Figure 2, means that only some code on that line executed. For example, a ternary expression like
var b = (a > 0) ? a : 0is colored both red and green.
To open the Coverage tab:
- Open the Command Menu.(Cmd+Shift+P (Mac) or Ctrl+Shift+P (Windows, Linux).)
- Start typing
Coverageand select Show Coverage.