The YouTube application available from the eShop is a web-browser. Like the main browser this uses WebKit. All data is downloaded with plaintext HTTP.
User-Agent and Browser Versions
The user-agent format is almost identical to the main browser: "Mozilla/5.0 (Nintendo 3DS; U; ; Factory Media Production) Version/<version>.<region>". <lang> is "en", "fr", etc. <region> is "US", "EU", etc. See below for <version>.
|Browser version||USA/EUR CDN Title-version||Notes|
|1.7498||v16 and v0||Initial app version.|
|1.7498||v1056 and v1040||First app update. This build uses the v7.0 NCCH encryption for the main CXI, this was the first retail title to use this.|
Looks like the YouTube application uses an upgraded version of Webkit. It has the ability to construct Blobs, of course the HTML5 video element, and possibly the Audio element (Untested). It also supports localStorage.
Inside all webkit applications for the 3DS, there is a pseudo-webserver running that links to a structure inside ExeFS. This also possiblly exists for the Internet Browser, but is unused.