There's no generic HTML import function yet. Right now you can only have a page pull in javascript (through a script tag or ES module import) or CSS files, but

There's no generic HTML import function yet. Right now you can only have a page pull in javascript (through a script tag or ES module import) or CSS files, but critically not component HTML files. So it's hard to have a component defined by HTML and CSS that can be shared and imported in other pages. There's no "grab this HTML page and give me the DOM to insert/clone/use how I please" in pure HTML. The closest thing is iframe but it's not really a general component tool. For better or worse JS is currently the way to share logic and fragments between HTML pages. There was a working group looking at HTML imports but I think it all got spun down.