You wouldn't, but shadowroot is bad design for that. What's wrong with bootstrap or any other number of of existing frameworks? I don't want to include a button

You wouldn't, but shadowroot is bad design for that. What's wrong with bootstrap or any other number of of existing frameworks? I don't want to include a button and have it stick out like a sore thumb because it ignores my page's styles. ::part() is a stupid hack to make up for a bad approach. If you want to include something "as-is" (i.e. the demos you mention), use iframes. If you want to include a component like a button, use any of the existing frameworks that work just fine. Trying to build a button that pretends to be a browser-native component but actually isn't is a terrible idea.