Very simple deno webserver mostly exactly from the example in the docs at https://deno.land/manual/examples/http_server

server.ts

// Start listening on port 8080 of localhost.
const server = Deno.listen({ port: 8080 })
console.log('HTTP webserver running...')

const headers = Object.freeze({
  'Content-type': 'plain/text'
})

for await (const conn of server) {
  // deno-lint-ignore no-extra-semi
  ;(async () => {
    const httpConn = Deno.serveHttp(conn)

    for await (const requestEvent of httpConn) {
      const body = `Your user-agent is:\n\n${requestEvent.request.headers.get('user-agent') ?? 'Unknown'}`
      const resp = new Response(body, { status: 200, headers })
      requestEvent.respondWith(resp)
    }
  })()
}