Is using proper grammar an unofficial rule of Reddit?

I'm new to Reddit and I am currently exploring some different communities. I noticed most people here use "proper" grammar regardless if the subreddit generally has a formal or casual tone. Along with this, people who speak more casually, such as not capitalizing sentences and using emojis, get downvoted a lot or ignored. Should I be careful and try to write more formally or is it not of major concern? Thank you :)