GraphML is the standard file format for yFiles for HTML. It is an XML format that provides
great flexibility when storing custom data. However, note that these attributes (such as
styles or even node locations) are not standardized, so you probably will not be able to
exchange all of them between different graph libraries, for example.
Built-in types, in particular styles, can be de-/serialized out of the box which is
illustrated in this application. Custom types can be de-/serialized automatically if they
meet specific rules that are described in
Automatically Serialized Types.