Beginners who are learning guitar solely via YouTube face several challenges. The first one that comes to mind is that no one is there to correct them when they do something wrong. Luckily, there's a YouTube video that addresses this issue!
Check out "Five Mistakes Beginners Make When Learning Guitar Online" by guitar instructor/author Andy Growley, better known in guitar circles as Andy Guitar. In the clip, Andy tells you exactly what to watch out for, including strumming by twisting your wrist and placing your chording fingers on the fretboard one at a time. Come on, you know you're guilty.