HTML5 Storage isn't supported. You'll need to either build a browser for Kobo or use a polyfill.
Alternatively, using LD_PRELOAD to inject the following code
might work:
Code:
auto s = QWebSettings::globalSettings();
s->setAttribute(QWebSettings::LocalStorageEnabled, true);
auto p = "/mnt/onboard/storage";
s->setOfflineStoragePath(p);
s->enablePersistentStorage(p);