Styling infowindow google maps

Im making a infowindow for my markers in google map and I want to have the window in a separated html file but I dont know how to reference it.
This is how I have done it:

  let content = `<h1>${marker.shipname}</h1>
        <p>Speed: ${marker.speed}</p>
        <p>Destination: ${marker.destination}</p>
        <p>Lastport: ${marker.lastport}</p>
        <p>Location: ${marker.location}</p>
        <p>Course: ${marker.course}</p>
        <p>Eta: ${marker.eta}</p>;

But want to have that style in a separate html.