What do I need in order to use an intermediate CA in an ssl certificate? My understanding is that the free account will not support it or even allow it, but I don’t see any description of which accounts do and do not support this.
InfinityFree - no support for intermediary CA.
XVHOST - full support to upload your intermediary CA.
I’m not sure what else you are looking for in terms of support. In short: a CA certificate helps explain to browsers why they should trust a certificate. Most browsers will trust all certificates by default, but some (esp. older) devices and browsers need a CA cert to know your certificate is trusted.