What's the best way to create reusable HTML components?

If a beginner web developer has to componentize her HTML page into different partials, what's the best way to do it. I know this can be done in React via components and in AngularJS via templates. Let's not jump into JS frameworks.

What is the most simple and basic way to create and use HTML partials like Header.html, footer.html, sidebar.html, etc?