Inside a load function (as well as in form actions, hooks and API routes, which we'll learn about later) you have access to a setHeaders function, which — unsurprisingly — can be used to set headers on the response.
Most commonly, you'd use it to customise caching behaviour with the Cache-Control response header, but for the sake of this tutorial we'll do something less advisable and more dramatic:
src/routes/+page.server.js
export function load({ setHeaders }) {
setHeaders({
'Content-Type': 'text/plain'
});
}(You may need to reload the iframe to see the effect.)