This host 185.26.156.231 offers a working http answer (200) for dns-no-tcp.klml.org

There are two DNS entries for Round-robin_DNS

The other host with IP 188.106.110.128 (this is a dialupIP) does not answer TCP/IP at all.

It seems browsers failover to the other, hopefully working host

cURL even shows me, it is doing the failover:

curl -vI http://dns-no-tcp.klml.org/
*   Trying 188.106.110.128...
* TCP_NODELAY set
* connect to 188.106.110.128 port 80 failed: No route to host
*   Trying 185.26.156.231...
* TCP_NODELAY set
* Connected to dns-no-tcp.klml.org (95.143.172.147) port 80 (#0)
> HEAD / HTTP/1.1
> Host: dns-no-tcp.klml.org
> User-Agent: curl/7.58.0
> Accept: */*
...

Try also this not working dns-http500.klml.org with two DNS entries with one with an http status error 500 answer.