Over the last two or three days, I've received several 503s while browsing through the devmanual, the dev handbook, and while loading the herds list.
Specific URLs that I can produce for you are:
Just so that I could confirm the "https://www.gentoo.org/proj/en/metastructure/herds/herds.xml" was the proper link, I attempted to load it and it has been attempting to load for several minutes now. (took about 4-5 minutes in the end)
Error 503 Service Unavailable
The version of www.gentoo.org you refer to is no longer in operation (it's wwwold. currently and in queue for discontinuation).
I checked devmanual, and can not find any consistent error pattern. The various XML runtimes we use sometimes fail, as long as these issues are only intermittent, there's nothing to fix.