SoftBank Mobile was previously Vodafone and before that J-Phone.
Markup
All SoftBank 3G phones support XHTMLBasic and separate CSS.
Carrier reference
Softbank has good japanese documentation on their creator site
HTTP Headers
Handset Capabilities
Mobile Creation Registration
Some of the content on SoftBank's developer site requires registration. The registration is free and can be done here. By registering you will also be allowed to post on their community site.