Glad you like the post! Testing covers a lot of ground in web development. The browser developer tools is the way to start where you can inspect the DOM, check the console for errors, track network requests, etc. You can then take it to the next step by adding automated tests to your code. I haven't done a lot of automated testing, so I can't recommend something specific. But I did hear names like Jest, Vitest, and Selenium.
Hope that helps!