This confused me for a bit here so I’m writing it out while it’s fresh in mind. Just because you’re using a web component doesn’t mean the styles of it are entirely isolated. You might have content within a web component that is styled normally along with the rest of your website. Like this:
See the Pen Web Component with Global Styles (because no Shadow DOM) by Chris Coyier (@chriscoyier) on CodePen.
And where you define your shadow DOM you use that header: