I’ve just submitted a bug report at freebsd.org.

Seems they’re shipping mod_proxy_html 3, but failing to install proxy_html.conf for their users.  That means it ships configured to do nothing, where mod_proxy_html 2.x would’ve done its job with W3C HTML or XHTML.  In other words, use it as a drop-in replacement for the previous version, and it stops working.

Add to that, they’re shipping version 3.0.0, which has a bug (fixed in 3.0.1) that segfaults in the absence of any ProxyHTMLLinks directive, and thus as-shipped with FreeBSD.

Any users wanting to fix this problem: all you have to do is grab the default proxy_html.conf and Include it into your httpd.conf, and mod_proxy_html is 3.x back to being a drop-in replacement for 2.x, but with new capabilities.