I'm not new to programming, but I am somewhat new to web development and I'm trying to figure out the most preferred way of taking a standalone header from one html document and adding it to other html documents without code duplication. If possible I want to do this with Javascript so I can learn with more basic tools before expanding further.

I've researched this a fair bit, but the advice is a bit confusing since it either seems out of date or possibly not the most secure way of handling things. Is there a preferred way of doing something like this?