Things with browsers are always changing, updating etc. It makes recording videos a dangerous decision, since almost anything I say may be outdated, or will be outdated soon. For instance, in the video you just watched, I used the prefixes for border-radius, but I am fairly certain that it is supported by most of the browsers now. I even went to https://caniuse.com to check it out.
But when I checked on column-count it showed that the prefixes are still needed.
And I didn't even talk about prefixes for the new Microsoft browser, Edge. Right now, if a property is standard there will be no need for any prefixes in Edge. If you are going to use experimental features, use the -ms prefix.
Don't forget, you never know which version of a browser someone may be using. Hopefully you want to share your work with as many people as possible. So plan on your page working on a few iterations back of any browser.