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CSS/JS Coverage in Chrome DevTools


Since version 59, Chrome DevTools ship with a coverage analyzer that will tell you how much of the Javascript you load is actually used on your pages.

This is important because, if the code is yours, you can eliminate dead weight and reduce the size of your site's styles and scripts.

Running Coverage Analysis #

Running coverage from Chrome DevTools is fairly straight forward. Open DevTools (Command + Option + I on Mac or Control + Shift + I on Windows) and look at the bottom of the DevTools window, there should be a coverage tab.

When you select the coverage tab you will see something like the figure below. Click on the reload icon to reload the page and analyze your site's coverage.

Chrome DevTools showing coverage panel
Chrome DevTools showing coverage panel

Chrome DevTools showing coverage panel

The coverage panel will present a list of all the scripts and styles loaded on the page along with information about each resource loaded.

Chrome DevTools showing the results of a coverage analysis
Chrome DevTools showing the results of a coverage analysis

Chrome DevTools showing the results of a coverage analysis

The information provides is listed below:

  • The URL of the resource that was analyzed
  • The Type of resource (CSS, JavaScript, or both)
  • The Total Bytes column shows the total size (in bytes) of the resource
  • The Unused Bytes column is the number of unused bytes for the resource, both as a number of bytes and a percentage
  • The last column shows a visualization of the Total Bytes and Unused Bytes columns
    • The red section of the bar is unused bytes
    • The green section is used bytes

Looking at the coverage table I can see that there are scripts and styles that are seldom used and can dig into what resources they are, whether I need them, and whether I can reduce them further.

I've asked the DevTools team on Twitter if it's possible to save the Coverage data as a CSV file to do further analysis offline.

Questions and Solutions #

Unsurprisingly the largest resources are those I don't have direct access to modify. I need to do more research to see if I can tree shake these external scripts and how to do it.

The biggest question is how to shrink the size of these packages that I have no access to. Is it a matter of using a lighter version, like the slim package for jQuery, or is it a matter of using replacement libraries if they exist?

Doing it in CSS is easier. You can use UNCSS or similar tools to eliminate unused CSS. When working with CMSs like WordPress the question becomes: How do you do create and inline the Critical CSS in production for all pages on your site that we may create hourly or more frequently? Do we do it when the page is visited?

This works best when you're building a site by hand or with a build system but how do you do it in a WordPress site or any other site that depends on a CMS?

As far as I know, there is no free tool that will create critical CSS and inline it on pages and posts of a WordPress-powered site; most of them are parts of paid plugins. There are tools that have a PHP implementation of UNCSS, but there's still much research to do to see how it can be worked on a WordPress site. It scripts that need to remain in the head on their own. AMP plugins are the best/worst example of this behavior… they will fail if they are placed at the bottom of the page.

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