The websites in the game can be loaded from either the disc, or the update:
On disc (PS3): ps3.rpf\data\cdimages\scaleform_web.rpf
*Use LibertyV to open RPF files.
The websites are made up of 2 files: one is a normal .ctd/.xtd texture file, and the other is a scaleform .gfx file.
.ctd/.xtd files can be opened with GTA V Console Texture Editor, and house DXT5 .dds files - which can be edited in paint.net/gimp/photoshop.
The .gfx is an uncompressed scaleform file, which can be converted to a .swf file by opening it in a hex editor and changing the file header from GFX to FWS. From there you can decompile the swf file using a decompiler like Trillex or SoThink SWF.
You can add a website, or change a website's domain name, just by renaming the .ctd/.xtd and .gfx files. So if I wanted to add a new site, I'd just take a site and rename it to www_sonofugly_com.gfx and .ctd/.xtd:
And then going to sonofugly.com in the in-game browser will turn up:
...And I also edited the header image.
The reason I looked into this is because I'm currently working on a handling mod for V, but testing hard to find cars can take quite a while without a trainer, so I had the idea to edit one of the in-game website that sells cars and make them sell rare, or even all, cars. However, I've never played around with SWF, or Flash, or ActionScript before so I'm a bit over my head... and it'd take along time to repack and test a website after editing to make sure I didn't screw something up and it still worked. It'll be interesting to see what kind of website can be made for V, perhaps something like whattheydonotwantyoutoknow.com, or maybe someone can make a website like I was going to do.