Uno R4 Wifi Access Point

HTML is not my strong point, but this code section is what prints the messages to the screen

            client.println("HTTP/1.1 200 OK");
            client.println("Content-type:text/html");
            client.println();

            // the content of the HTTP response follows the header:
            client.print("<p style=\"font-size:7vw;\">Click <a href=\"/H\">here</a> turn the LED on<br></p>");
            client.print("<p style=\"font-size:7vw;\">Click <a href=\"/L\">here</a> turn the LED off<br></p>");
            
            // The HTTP response ends with another blank line:
            client.println();

Basically it creates the HTML code required by "printing" to the webserver, ie the client

Try this crude example in place of the code above

            client.println("HTTP/1.1 200 OK");
            client.println("Content-type:text/html");
            client.println();

            // the content of the HTTP response follows the header:
            client.print("<p style=\"font-size:7vw;\">Click <a href=\"/H\">here</a> turn the LED on<br></p>");
            client.print("<p style=\"font-size:7vw;\">Click <a href=\"/L\">here</a> turn the LED off<br></p>");
            
            // The HTTP response ends with another blank line:
            client.println();
            client.println("HTTP/1.1 200 OK");
            client.println("Content-type:text/html");
            client.println();

            // the content of the HTTP response follows the header:
            client.println("Hello World");
            // client.print("<p style=\"font-size:7vw;\">Click <a href=\"/H\">here</a> turn the LED on<br></p>");
            // client.print("<p style=\"font-size:7vw;\">Click <a href=\"/L\">here</a> turn the LED off<br></p>");
            
            // The HTTP response ends with another blank line:
            client.println();

Then go and read up on HTML to do it properly !