There is an AMP HTML framework originally from Google concerned with website usability particularly on mobiles. To push people towards this framework Google include Lighthouse as a dev tool inside their Chrome browser.

To blindly follow what such tools recommend is not sensible. Everything depends on the website and who uses it. But one Lighthouse change I have made is to inline the CSS for my personal website. This should improve the speed accessing uncached pages for the website, and the cost of adding a few thousand bytes of CSS should not be major. This isn't so true if a webpage contains 100KBs of inline CSS, that is just penalising anyone looking at more than a single page. But that isn't my situation.

The Liquid templating code strives to only inline CSS required for each page.