Insert HTML to Jekyll post

Actually, if you are using markdown to create your posts, you can use Liquid's include tag.

You just have to create a folder with the name _includes/ in the DocumentRoot of your project, then, create an HTML file inside, for example "mycomponent.html" and call it in your post with something like this:

{% include mycomponent.html %}

This file can have any HTML/CSS/JS you want.

Notice how you include just your HTML file; the external CSS/JS can be included in the layout that you're using.