Surely it'd make more sense for the name to be one of the following:
I'm sure there must be some reason and I'd hate to think it's now set in stone just because no one questioned this at the time. Is there another naming convention that I'm unaware of?
Interestingly enough, Microsoft first called it
IXMLHTTPRequest when it was first added to the MSXML library.
It was Mozilla that used the name
XMLHttpRequest when it added the concept into Gecko, implementing the idea to mimic the MS interface. It has since become the defacto standard, tying all other implementations to Mozilla's decision.
You'd have to go spelunking in the Mozilla Bugzilla to see if you can find any reasoning for the caps change there, but I suspect that not much thought went into it and the lowercasing of the
ttp part is accidental.
This is corroborated by the misspelling of the Microsoft interface in the nsIXMLHttpRequest interface definition (earliest revision in the Mozilla Mercurial repository):
Mozilla's XMLHttpRequest is modelled after Microsoft's IXMLHttpRequest object. The goal has been to make Mozilla's version match Microsoft's version as closely as possible, but there are bound to be some differences.
Some naming guidelines make a distinction between "short" and "long" acronyms. For instance, the coding style guide for Microsoft's .Net runtime specifies that short acronyms should be in block caps while long acronyms should only have the first letter capitalized. Their threshold for a long acronym is 3 letters, so would favor "XmlHttpRequest", however it is not unreasonable to think some people may use a similar rule with 4 characters as the threshold.
I've looked at old copies of the mozilla.org style guide, and none seem to specify anything about acronyms, but it's possible that either an older Netscape guide did, or that the developer was applying a rule he'd picked up elsewhere.