I added a paragraph at the beginning which clarifies who this guide is aimed at (from my perspective, at least) and a few words of caution about how readers should interpret these suggestions.
It also mentions the ES6 features mentioned in the JavaScript section. While I agree ES6 is important, I'd say that the majority of newer users will not implicitly understand that you are using unsupported features. It's good to mention it, at least.
I added a paragraph at the beginning which clarifies who this guide is aimed at (from my perspective, at least) and a few words of caution about how readers should interpret these suggestions.
It also mentions the ES6 features mentioned in the JavaScript section. While I agree ES6 is important, I'd say that the majority of newer users will not implicitly understand that you are using unsupported features. It's good to mention it, at least.