What is helium-css?

Helium is a browser-based tool for finding CSS rules that are written in your stylesheets but never actually used on any page of your website. Over time, stylesheets tend to accumulate rules left over from old features, deleted components, or redesigns. Helium helps you identify which of those rules are safe to remove. The way it works is straightforward: you add a small script tag to your site so that Helium loads alongside your pages during testing. Then, in the Helium interface that appears, you paste a list of URLs from different sections of your site and click Start. Helium visits each page, reads through your stylesheets, and checks whether each CSS selector it finds is actually matched by any element on any of the pages you listed. When it finishes, it shows you a report broken down by stylesheet. The results use color-coding to help you prioritize. Green marks selectors that were not matched on any page and are the main candidates for removal. Black marks selectors that appeared alongside unmatched ones in grouped rules, so you can find them easily in your code. Red marks selectors the browser could not parse at all. Blue marks selectors that require user interaction to trigger, like hover or focus states, which Helium cannot test automatically. There are a few constraints worth knowing before you start. Helium can only check stylesheets hosted on the same domain as the pages being tested because of browser security restrictions. Any JavaScript errors on your pages will also interrupt the scan, so it helps to verify your pages are error-free beforehand. The URL list needs to be formatted with one URL per line. The README is clear that you should only run this tool on a local or private version of your site, not on a live public one, since the Helium interface will be visible to anyone who loads the page while it is active.